Support equirectangular cubemap projections

Most of the environemnt cubemaps one can find online are in equirectangular format. Its impractical to expect people to convert those to 6-cubemap images before they can be turned into a ktx file.

IIRC there is a way to extract data for each side from the equirectangular projection onto each cubemap face without having to render it like a 3D engine would do. It will be nice if this is supported.

The issue names no source files, tests, or entry points. Start by locating the existing cubemap-to-KTX conversion path and any projection-related tests; done means an equirectangular environment map can be converted into the expected six cubemap faces and packaged as KTX without separate preprocessing.

Written by the indexing model from the issue text.