BC6 encoder

[RunDevelopment]

Make a BC6 encoder, both signed and unsigned. This will complete the library in the sense that all formats can be decoded and encoded (except for ASTC formats #23).

This should be rather doable after #86 is in. BC6 and BC7 share some concepts, so a lot of the ideas the BC7 encoder uses will likely also be useful for the BC6 encoder.

References:

• Specification