wgpu: Specify which cube face is being written to #285

Job	Run time
build (windows, x86_64, msvc, Debug)	1m 3s
build (windows, x86_64, msvc, Release)	52s
build (windows, aarch64, msvc, Debug)	2m 6s
build (windows, aarch64, msvc, Release)	2m 12s
build (linux, x86_64, gcc, Debug)	32s
build (linux, x86_64, gcc, Release)	33s
build (linux, x86_64, clang, Debug)	52s
build (linux, x86_64, clang, Release)	1m 45s
build (linux, aarch64, gcc, Debug)	1m 20s
build (linux, aarch64, gcc, Release)	1m 27s
build (linux, aarch64, clang, Debug)	1m 17s
build (linux, aarch64, clang, Release)	1m 58s
build (macos, aarch64, clang, Debug)	1m 1s
build (macos, aarch64, clang, Release)	1m 1s
	17m 59s

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