The cmgen tool can now generate KTX files.
The cmgen tool can now generate KTX files.

For each envmap/skybox pair, this will reduce the number of downloads
from 61 files to 3 files.

Note that we are still using sh.txt for coefficients, I'll see if it
would be easy to embed that in the KTX metadata.

Also, I will add KTX support to roughness-prefilter soon.

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