Investigate GDeflate supercompression

- Low-hanging fruit is to support this scheme as a general supercompression scheme.
- Then, it may be interesting to combine this approach with BasisLZ.

https://github.com/microsoft/DirectStorage/tree/main/GDeflate
https://github.com/NVIDIA/libdeflate/tree/gdeflate

Start by reviewing the linked Microsoft DirectStorage GDeflate material and NVIDIA libdeflate GDeflate branch. Determine what supporting GDeflate as a general supercompression scheme would require in the KTX specification, and separately assess whether combining it with BasisLZ is in scope. Done should be a settled scope and specification plan.

Written by the indexing model from the issue text.