mesa: Enable compute-based transcoding to DXT5
By enabling this path, we get a 56% decrease in upload time on a texture upload microbenchmark. This was measured on an Ice Lake with an iris driver that tries to use the compressed format fallback path.
Based on the BC3 encoder from the MIT-licensed GPU compressor, Betsy: https://github.com/darksylinc/betsy.
Edited by Nanley Chery