This no longer converts from sRGB to linear for the reference mip
downsample - even if the original mipmap creation tool used an sRGB
colorspace (which isn't really guaranteed, and may even change per
game), this is a "fast" heuristic that's only an estimate anyway.
The average diff is also now stored in a u64, avoiding floating point
calculations in the per-pixel hot loop.
This should speed up the detection significantly, hopefully fixing
jank when loading in new textures.
Jonathan Hamilton
ac05d1a593