Add support for texture flipping of BC6 and BC7 formats

[tonihele]

Add support for texture flipping of BC6 and BC7 formats. This is definitely not an easy task. It might be doable. Fun with bits.

Here is some pondering on the subject:
https://gamedev.stackexchange.com/questions/35379/how-to-flip-a-bc6-bc7-texture

See #1619 for the BC6 and BC7 support implementation.

[stephengold]

Thanks for opening an issue so this won't be forgotten.