ImGui integration prototype #10184

Job	Run time
build_test_headless_alpine	8m 27s
build-windows (x64)	9m 52s
build-uwp	8m 8s
build (ubuntu-latest, test, clang, clang++, ./b.sh --headless --unittest, clang-normal)	3m 19s
build-windows (ARM64)	9m 53s
build (ubuntu-latest, gcc, g++, ./b.sh --headless --unittest, gcc-normal)	3m 19s
build (ubuntu-latest, android, clang, clang++, cd android && ./ab.sh -j2 APP_ABI=arm64-v8a UNITTE...	2m 54s
build (ubuntu-latest, android, clang, clang++, cd android && ./ab.sh -j2 APP_ABI=armeabi-v7a UNIT...	3m 6s
build (ubuntu-latest, android, clang, clang++, cd android && ./ab.sh -j2 APP_ABI=x86_64 UNITTEST=...	2m 43s
build (ubuntu-latest, android, clang, clang++, cd android && ./ab.sh -j2 APP_ABI=arm64-v8a OPENXR...	2m 48s
build (ubuntu-latest, android, clang, clang++, ./b.sh --libretro_android ppsspp_libretro, android...	4m 58s
build (ubuntu-latest, qt, gcc, g++, ./b.sh --qt, qt)	3m 39s
build (ubuntu-latest, libretro, gcc, g++, make -C libretro -f Makefile -j2, gcc-libretro)	2m 42s
build (ubuntu-latest, libretro, clang, clang++, make -C libretro -f Makefile -j2, clang-libretro)	2m 40s
build (macos-latest, test, clang, clang++, ./b.sh --headless --unittest --fat --no-png --no-sdl2,...	8m 35s
build (macos-latest, ios, clang, clang++, ./b.sh --ios, ios)	4m 14s
test	0s
test-windows	40s
	1h 21m 57s

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