Squashed 'external/imgui/' content from commit 02e9b8cac

git-subtree-dir: external/imgui
git-subtree-split: 02e9b8cacde285c5f9040adf212703cc3810bd97

examples/example_win32_opengl3/build_mingw.bat

@@ -0,0 +1,8 @@
+@REM Build for MINGW64 or 32 from MSYS2.
+@set OUT_DIR=Debug
+@set OUT_EXE=example_win32_opengl3
+@set INCLUDES=-I../.. -I../../backends
+@set SOURCES=main.cpp ../../backends/imgui_impl_opengl3.cpp ../../backends/imgui_impl_win32.cpp ../../imgui*.cpp
+@set LIBS=-lopengl32 -lgdi32 -ldwmapi
+mkdir %OUT_DIR%
+g++ -DUNICODE %INCLUDES% %SOURCES% -o %OUT_DIR%/%OUT_EXE%.exe --static -mwindows %LIBS% %LIBS%