status of basis ETC1 support

I've transitioned my 2D-only prototype to a full-blown class now, instead of it living in my experimental framework as a huge function.

Next up are things like macroblock support, more endpoint/selector codebook refinements, an investigation into alternative selector compression schemes, and an experiment to exploit endpoint/selector codebook entry correlation. After this, I'm rewriting the code so it works on texture arrays, cubemaps, etc.

This rewritten new code will be the "front end" of the full ETC1 compressor. The back end (that does the coding) comes after the front end is in good shape. Unlike crunch, basis will use the same basic front end for both .RDO mode and .CRN (or .basis) mode.

This compressor is also compatible with the ETC1 "subset" format I mentioned here, which means it could be trivially transcoded to DXT1 with the aid of a precomputed lookup table.