Move compression backend to Compressonator

[cdwfs]

Motivation:

• ispc_texcomp hasn't been updated in years, including merging some PRs for significant-looking bugfixes. Compressonator is under active development (for now).
• ispc_texcomp requires a fancy custom compiler, which complicates the build process and hinders both distribution and CI/testing. Compressonator can be easily built from source on any platform with a relatively modern C++ compiler.
• Compressonator supports more output formats than ispc_texcomp.

[cdwfs]

A functional Compressonator port is in the https://github.com/cdwfs/img2ktx/tree/compressonator branch. Compressonator requires cloning an enormous pile of dependency, 99% of which are unnecessary for the library-only build. Wait until https://github.com/GPUOpen-Tools/Compressonator/issues/58 is resolved before merging into master.

[cdwfs]

Current workaround: put local copies of the sole external dependency (half.cpp & friends from ilmbase) in the third_party directory, and configure Compressonator to look there instead of in its own Ext directory. Recursive submodule clone is no longer necessary.

[cdwfs]

As of Compressonator 3.2, the build headaches have been resolved; I'll merge this branch into master shortly after a bit more local testing.