Merge branch 'examples_refactor2' into viewport (#1870)

# Conflicts:
#	examples/README.txt
#	examples/directx11_example/main.cpp
#	examples/directx12_example/main.cpp
#	examples/imgui_impl_dx10.cpp
#	examples/imgui_impl_dx11.cpp
#	examples/imgui_impl_dx12.cpp
#	examples/imgui_impl_glfw.cpp
#	examples/imgui_impl_opengl2.cpp
#	examples/imgui_impl_opengl2.h
#	examples/imgui_impl_opengl3.cpp
#	examples/imgui_impl_sdl2.cpp
#	examples/imgui_impl_sdl2.h
#	examples/imgui_impl_vulkan.cpp
#	examples/imgui_impl_vulkan.h
#	examples/imgui_impl_win32.cpp
#	examples/imgui_impl_win32.h
#	examples/opengl2_example/main.cpp
#	examples/opengl3_example/main.cpp
#	examples/sdl_opengl2_example/main.cpp
#	examples/sdl_opengl3_example/main.cpp
#	examples/sdl_vulkan_example/main.cpp
#	examples/vulkan_example/main.cpp
#	imgui.cpp
#	imgui.h

examples/sdl_opengl3_example/main.cpp

@@ -42,7 +42,7 @@ int main(int, char**)
     io.ConfigFlags |= ImGuiConfigFlags_ViewportsNoTaskBarIcons;
     //io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;  // Enable Keyboard Controls
 
-    ImGui_ImplSDL2_Init(window, gl_context);
+    ImGui_ImplSDL2_InitForOpenGL(window, gl_context);
     ImGui_ImplOpenGL3_Init();
 
     // Setup style