gltfpack: Always use zstd compression with UASTC
gltfpack: Always use zstd compression with UASTC

This provides a substantial reduction in image size; the compression
time cost is under 10% compared to existing UASTC codec even given a
relatively high zstd level (level 9 gives ~10% smaller results compared
to default level 3).

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer