Add: KHR_materials_variants #361

Job	Run time
build_linux (gcc-9, NO)	2m 18s
build_linux_deprecated_extensions (gcc-9)	2m 8s
build_linux_deprecated_extensions (gcc-10)	2m 39s
build_linux (gcc-10, NO)	2m 7s
build_linux_deprecated_extensions (clang-13)	2m 33s
build_linux (gcc-10, YES)	2m 25s
build_windows (NO)	4m 11s
build_windows_deprecated_extensions	3m 7s
build_linux (clang-13, NO)	2m 29s
build_linux (clang-15, NO)	2m 33s
build_windows (YES)	4m 14s
build_linux (clang-15, YES)	2m 26s
	33m 10s

Provide feedback