Intra-Residual Weight Grid DCT/DST for BC7/ASTC

This GPU texture supercompression method for weights (or "indices", or "selectors") uses neighboring blocks to predict the weight grid of the current block, before applying a forward 2D DCT/DST to the weights, quantizing the coefficients, and coding them. The current working prototype (which I've been showing live on X) creates weight predictions using weights from a small neighborhood of already…