Backends: OpenGL3: Embed our own minimal GL loader (amends). (#4445)

docs/CHANGELOG.txt

@@ -54,6 +54,7 @@ Breaking Changes:
   This is unfortunately a global SDL setting, so enabling it _might_ have a side-effect on your application.
   It is unlikely to make a difference, but if your app absolutely needs to ignore the initial on-focus click:
   you can ignore SDL_MOUSEBUTTONDOWN that events coming right after a SDL_WINDOWEVENT_FOCUS_GAINED event).
+- Backends: OpenGL3: added a third source file "imgui_impl_opengl3_loader.h". [@rokups]
 - Internals: (for custom widgets): because disabled items now sets HoveredId, if you want custom widgets to
   not react as hovered when disabled, in the majority of use cases it is preferable to check the "hovered"
   return value of ButtonBehavior() rather than (HoveredId == id).
@@ -119,6 +120,10 @@ Other Changes:
 - Backends: DX9: Explicitly disable texture state stages after >= 1. (#4268) [@NZJenkins]
 - Backends: DX12: Fix texture casting crash on 32-bit systems (introduced on 2021/05/19 and v1.83) + added comments
   about building on 32-bit systems. (#4225) [@kingofthebongo2008]
+- Backends: OpenGL3: Embed our own minimal GL headers/loader (imgui_impl_opengl3_loader.h) based on gl3w.
+  Reduces the frequent issues and confusion coming from having to support multiple loaders and requiring users to use and
+  initialize the same loader as the backend. [@rokups]
+  Removed support for gl3w, glew, glad, glad2, glbinding2, glbinding3 (all now unnecessary).
 - Backends: OpenGL3: Handle GL_CLIP_ORIGIN on <4.5 contexts if "GL_ARB_clip_control" extension is detected. (#4170, #3998)
 - Backends: OpenGL3: Destroy vertex/fragment shader objects right after they are linked into main shader. (#4244) [@Crowbarous]
 - Backends: OpenGL3: Use OES_vertex_array extension on Emscripten + backup/restore current state. (#4266, #4267) [@harry75369]
@@ -126,13 +131,11 @@ Other Changes:
 - Backends: OSX: Added a fix for shortcuts using CTRL key instead of CMD key. (#4253) [@rokups]
 - Examples: DX12: Fixed handling of Alt+Enter in example app (using swapchain's ResizeBuffers). (#4346) [@PathogenDavid]
 - Examples: DX12: Removed unecessary recreation of backend-owned device objects when window is resized. (#4347) [@PathogenDavid]
+- Examples: OpenGL3+GLFW,SDL: Remove include cruft to support variety of GL loaders (no longer necessary). [@rokups]
 - Examples: OSX+OpenGL2: Fix event forwarding (fix key remaining stuck when using shortcuts with Cmd/Super key).
   Other OSX examples were not affected. (#4253, #1873) [@rokups]
 - Examples: Updated all .vcxproj to VS2015 (toolset v140) to facilitate usage with vcpkg.
 - Examples: SDL2: Accomodate for vcpkg install having headers in SDL2/SDL.h vs SDL.h.
-- Examples: OpenGL3+GLFW/SDL: Added minimal OpenGL loader imgui_impl_opengl3_loader.h and use it in OpenGL3 backend.
-  Support for using custom OpenGL loader in the backend is removed, IMGUI_IMPL_OPENGL_LOADER_XX (except ES2/ES3) are
-  no longer used. [@rokups]
 
 
 -----------------------------------------------------------------------

docs/EXAMPLES.md

@@ -117,21 +117,18 @@ GLFW (Mac) + Metal example. <BR>
 [example_glfw_opengl2/](https://github.com/ocornut/imgui/blob/master/examples/example_glfw_opengl2/) <BR>
 GLFW + OpenGL2 example (legacy, fixed pipeline). <BR>
 = main.cpp + imgui_impl_glfw.cpp + imgui_impl_opengl2.cpp <BR>
-**DO NOT USE OPENGL2 CODE IF YOUR CODE/ENGINE IS USING MODERN OPENGL (SHADERS, VBO, VAO, etc.)** <BR>
-**Prefer using OPENGL3 code (with gl3w/glew/glad/glad2/glbinding, you can replace the OpenGL function loader)** <BR>
+**DO NOT USE THIS IF YOUR CODE/ENGINE IS USING MODERN OPENGL (SHADERS, VBO, VAO, etc.)** <BR>
 This code is mostly provided as a reference to learn about Dear ImGui integration, because it is shorter.
 If your code is using GL3+ context or any semi modern OpenGL calls, using this renderer is likely to
 make things more complicated, will require your code to reset many OpenGL attributes to their initial
 state, and might confuse your GPU driver. One star, not recommended.
 
 [example_glfw_opengl3/](https://github.com/ocornut/imgui/blob/master/examples/example_glfw_opengl3/) <BR>
-GLFW (Win32, Mac, Linux) + OpenGL3+/ES2/ES3 example (programmable pipeline). <BR>
+GLFW (Win32, Mac, Linux) + OpenGL3+/ES2/ES3 example (modern, programmable pipeline). <BR>
 = main.cpp + imgui_impl_glfw.cpp + imgui_impl_opengl3.cpp <BR>
 This uses more modern OpenGL calls and custom shaders. <BR>
+This may actually also work with OpenGL 2.x contexts! <BR>
 Prefer using that if you are using modern OpenGL in your application (anything with shaders).
-(Please be mindful that accessing OpenGL3+ functions requires a function loader, which are a frequent
-source for confusion for new users. We use a loader in imgui_impl_opengl3.cpp which may be different
-from the one your app normally use. Read imgui_impl_opengl3.h for details and how to change it.)
 
 [example_glfw_vulkan/](https://github.com/ocornut/imgui/blob/master/examples/example_glfw_vulkan/) <BR>
 GLFW (Win32, Mac, Linux) + Vulkan example. <BR>
@@ -167,7 +164,6 @@ SDL2 (Mac) + Metal example. <BR>
 SDL2 (Win32, Mac, Linux etc.) + OpenGL example (legacy, fixed pipeline). <BR>
 = main.cpp + imgui_impl_sdl.cpp + imgui_impl_opengl2.cpp <BR>
 **DO NOT USE OPENGL2 CODE IF YOUR CODE/ENGINE IS USING MODERN OPENGL (SHADERS, VBO, VAO, etc.)** <BR>
-**Prefer using OPENGL3 code (with gl3w/glew/glad/glad2/glbinding, you can replace the OpenGL function loader)** <BR>
 This code is mostly provided as a reference to learn about Dear ImGui integration, because it is shorter.
 If your code is using GL3+ context or any semi modern OpenGL calls, using this renderer is likely to
 make things more complicated, will require your code to reset many OpenGL attributes to their initial
@@ -177,10 +173,7 @@ state, and might confuse your GPU driver. One star, not recommended.
 SDL2 (Win32, Mac, Linux, etc.) + OpenGL3+/ES2/ES3 example. <BR>
 = main.cpp + imgui_impl_sdl.cpp + imgui_impl_opengl3.cpp <BR>
 This uses more modern OpenGL calls and custom shaders. <BR>
-Prefer using that if you are using modern OpenGL in your application (anything with shaders).
-(Please be mindful that accessing OpenGL3+ functions requires a function loader, which are a frequent
-source for confusion for new users. We use a loader in imgui_impl_opengl3.cpp which may be different
-from the one your app normally use. Read imgui_impl_opengl3.h for details and how to change it.)
+This may actually also work with OpenGL 2.x contexts! <BR>
 
 [example_sdl_vulkan/](https://github.com/ocornut/imgui/blob/master/examples/example_sdl_vulkan/) <BR>
 SDL2 (Win32, Mac, Linux, etc.) + Vulkan example. <BR>

docs/TODO.txt

@@ -391,7 +391,6 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - backends: opengl: rename imgui_impl_opengl2 to impl_opengl_legacy and imgui_impl_opengl3 to imgui_impl_opengl? (#1900)
  - backends: opengl: could use a single vertex buffer and glBufferSubData for uploads?
  - backends: opengl: explicitly disable GL_STENCIL_TEST in bindings.
- - backends: opengl: consider gl_lite loader https://github.com/ApoorvaJ/Papaya/blob/3808e39b0f45d4ca4972621c847586e4060c042a/src/libs/gl_lite.h
  - backends: vulkan: viewport: support for synchronized swapping of multiple swap chains.
  - backends: bgfx: https://gist.github.com/RichardGale/6e2b74bc42b3005e08397236e4be0fd0
  - backends: mscriptem: with refactored examples, we could provide a direct imgui_impl_emscripten platform layer (see eg. https://github.com/floooh/sokol-samples/blob/master/html5/imgui-emsc.cc#L42)