Add skybox support #198

Job	Run time
build (macos-latest, clang, clang++, Release)	4m 24s
build (macos-latest, clang, clang++, Debug)	4m 17s
build (ubuntu-latest, gcc, g++, Debug)	6m 10s
build (windows-2022, cl, msvc, Debug)	4m 1s
build (windows-2022, cl, msvc, Release)	4m 42s
build (ubuntu-latest, gcc, g++, Release)	6m 18s
	29m 52s

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