Support cubemap cross single image source for cubemap generation

It is nicer to work with assets when intermediate files are single files instead of six files.
For example Nvidia texture tools exporter supports using single cubemap cross image as source for cubemap.

The issue names no files, tests, or entry points. Start by locating the cubemap-generation workflow and reviewing how it currently accepts six image files; done means accepting a single cubemap-cross image as the source and producing the equivalent cubemap.

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