Merge branch 'master' into docking

# Conflicts:
#	examples/example_apple_opengl2/main.mm
#	examples/example_emscripten/main.cpp
#	examples/example_glfw_opengl2/main.cpp
#	examples/example_glfw_opengl3/main.cpp
#	examples/example_glfw_vulkan/main.cpp
#	examples/example_glut_opengl2/main.cpp
#	examples/example_sdl_opengl2/main.cpp
#	examples/example_sdl_opengl3/main.cpp
#	examples/example_sdl_vulkan/main.cpp
#	examples/example_win32_directx10/main.cpp
#	examples/example_win32_directx11/main.cpp
#	examples/example_win32_directx12/main.cpp
#	examples/example_win32_directx9/main.cpp
#	examples/imgui_impl_glfw.cpp
#	examples/imgui_impl_sdl.cpp
#	imgui.cpp
#	imgui_widgets.cpp

docs/CHANGELOG.txt

@@ -153,6 +153,8 @@ Other Changes:
   support 32-bits indices. Most examples back-ends have been modified to support the VtxOffset field.
 - ImDrawList: Added ImDrawCmd::IdxOffset value, equivalent to summing element count for each draw command.
   This is provided for convenience and consistency with VtxOffset.
+- ImDrawCallback: Allow to override the signature of ImDrawCallback by #define-ing it. This is meant to
+  facilitate custom rendering back-ends passing local render-specific data to the draw callback.
 - ImFontAtlas: FreeType: Added RasterizerFlags::Monochrome flag to disable font anti-aliasing. (#2545)
   Combine with RasterizerFlags::MonoHinting for best results.
 - ImFontGlyphRangesBuilder: Fixed unnecessarily over-sized buffer, which incidentally was also not