Merge branch 'master' into viewport

# Conflicts:
#	examples/opengl2_example/imgui_impl_glfw_gl2.cpp
#	examples/opengl3_example/imgui_impl_glfw_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp
#	examples/sdl_opengl3_example/imgui_impl_sdl_gl3.h
#	examples/vulkan_example/imgui_impl_glfw_vulkan.cpp

examples/imgui_impl_glfw.h

@@ -11,6 +11,10 @@
 // If you are new to ImGui, see examples/README.txt and documentation at the top of imgui.cpp.
 // https://github.com/ocornut/imgui
 
+// About GLSL version:
+// The 'glsl_version' initialization parameter defaults to "#version 150" if NULL.
+// Only override if your GL version doesn't handle this GLSL version. Keep NULL if unsure!
+
 struct GLFWwindow;
 
 IMGUI_API bool        ImGui_ImplGlfw_InitForOpenGL(GLFWwindow* window, bool install_callbacks);