XBC7 planning

I've been doing this for fun as a side project, as ASTC is still way more important. (ASTC is the most deployed GPU format in the world, BC7 is niche by comparison.) It'll all be entirely open source:

- XBC7 v1 (completed, integrating ongoing): Always lossless for mode config+RGBA endpoints, lossless or lossy weights using either lossless residual DPCM or lossy absolute or residual DCT.

DPCM endpoint compression decorrelates G from R/B, properly taking into account dual plane modes (4/5). This does help.

DCT quant tables have been heavily tuned to be rotationally invariant and the very lowest H/V frequencies are now strongly protected at low Q's (i.e. tuned for 4x4). ~1.5-5.6 bpp, optionally threaded decoding, no BC7 format limits (all features, all partition patterns, 2/3 subsets etc.), near-lossless transcoding ASTC LDR 4x4, optionally threaded decoding (row strips - compressed seek table appears at start of file). Also supports transcoding to a bunch of old legacy formats using our existing real-time encoders: ETC using etc1f, PVRTC1, BC1, etc.

Encoder has a tone of unexploited headroom: Lagrangian RDO is possible. It's currently purposely quite conservative.

Future plan:

- XBC7 v2: Material stack support: 2-3 BC7 textures encoded simultaneously, correlations between textures exploited, still full transcoding to ASTC LDR 4x4. Will result in a large drop in average bpp for correlated PBR textures.

Will be able to use unified commands and shared weight grids between the textures in a material stack. Should be relatively easy to do.

- CUDA decoding of individual tiles in parallel (relatively easy, tile decoding is all-int and deterministic)

- Eventual HLSL port of CUDA decoder (will be painful and not looking forward to this)

When I show what we're doing with GPU textures to image/video codec specialists, they reply "this is totally obvious stuff - why have you guys not been using DCT/DST transforms for years already? Where have you been?"