Examples: Win32+OpenGL3: added batch. file for msys2/mingw build. (#6544)

examples/example_win32_opengl3/build_mingw.bat

+@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%