Add basic gradient sky dome #267

Job	Run time
build (stable, windows-latest)	4m 18s
build (nightly, ubuntu-latest)	2m 42s
build (stable, ubuntu-latest)	2m 42s
build (nightly, windows-latest)	4m 3s
build (nightly, macos-latest)	2m 1s
build (stable, macos-latest)	2m 8s
	17m 54s

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