Big example binding refactor (manually imported from Viewport branch, stripped out of Viewport code). (#1870)

Read examples/README.txt for some details.
ImDrawData: Added DisplayPos, DisplaySize fields honored by all backends (not strictly necessary to honor just now, but doing it to be ahead)

examples/imgui_impl_opengl2.h

@@ -0,0 +1,24 @@
+// ImGui Renderer for: OpenGL2 (legacy OpenGL, fixed pipeline)
+// This needs to be used along with a Platform Binding (e.g. GLFW, SDL, Win32, custom..)
+
+// Implemented features:
+//  [X] User texture binding. Use 'GLUint' OpenGL texture identifier as void*/ImTextureID. Read the FAQ about ImTextureID in imgui.cpp.
+
+// **DO NOT USE THIS CODE IF YOUR CODE/ENGINE IS USING MODERN OPENGL (SHADERS, VBO, VAO, etc.)**
+// **Prefer using the code in imgui_impl_opengl3.cpp**
+// This code is mostly provided as a reference to learn how ImGui integration works, because it is shorter to read.
+// If your code is using GL3+ context or any semi modern OpenGL calls, using this is likely to make everything more
+// complicated, will require your code to reset every single OpenGL attributes to their initial state, and might
+// confuse your GPU driver. 
+// The GL2 code is unable to reset attributes or even call e.g. "glUseProgram(0)" because they don't exist in that API.
+
+IMGUI_API bool  ImGui_ImplOpenGL2_Init();
+IMGUI_API void  ImGui_ImplOpenGL2_Shutdown();
+IMGUI_API void  ImGui_ImplOpenGL2_NewFrame();
+IMGUI_API void  ImGui_ImplOpenGL2_RenderDrawData(ImDrawData* draw_data);
+
+// Called by Init/NewFrame/Shutdown
+IMGUI_API bool  ImGui_ImplOpenGL2_CreateFontsTexture();
+IMGUI_API void  ImGui_ImplOpenGL2_DestroyFontsTexture();
+IMGUI_API bool  ImGui_ImplOpenGL2_CreateDeviceObjects();
+IMGUI_API void  ImGui_ImplOpenGL2_DestroyDeviceObjects();