New BC7 CPU encoder

[elasota]

New BC7 CPU encoder, significantly improves speed and quality.

Uses a mix of existing techniques (PCA + k-means clustering/LLS refinement) and a new one that tries to trade index precision for endpoint precision, and processes 8 blocks at once using SSE2.

Based on benchmarks against every other BC7 compressor that I'm aware of (both existing DirectXTex encoders, FasTC, Intel ISPC Texture Compressor, NVTT, Compressonator, bc7_gpu), it's roughly tied for best quality with NVTT and Compressonator, but is about 35x as fast as both of those and 10x as fast as the existing BC7 CPU encoder.

[walbourn]

Thanks. I appreciate the work here and the contribution. I do need to take some time to review it in detail, and I'm a little concerned about the comingling of various licenses here (i.e. it's just not MIT).

[elasota]

The Squish license and the Convection license are both MIT, so it's all one license.

[walbourn]

What exactly came from libsquish?

[elasota]

Just the initial endpoint selector code (a.k.a. the EndpointSelectorRGB and EndpointSelectorRGBA).

It's just 8-round Power Method so it can probably be rewritten if that's a problem.

[elasota]

Replaced the Squish dependency with a fresh implementation and fixed channel weighting in the process, merged in an important speed optimization. I think that should be it for any significant changes until review.

[Guekka]

Is this PR still in progress? It would be very useful on Linux

Edit: I just tried it. It is indeed way faster than the current version, but still 4-10 times slower than bc7_enc_rdo. So I will keep using the latter