In ResampleEffect, optimize the bilinear weights on a global scale.

In ResampleEffect, optimize the bilinear weights on a global scale.

In addition to the individual weight optimization we do when combining samples,
this technique optimizes the weights as a whole, through some linear algebra.
This means it can take into account effects such as multiple bilinear samples
influencing the same coefficient (which normally should not happen, but might
nevertheless due to imprecisions in the stored texture coordinates), or
non-combined sample positions that can't hit the exact middle of the texel.

In practical tests, this is extremely effective; it often reduces the computed
sum of squared coefficient errors by as much as a factor 1000, although I
haven't verified how often it actually saves us from having to do fp32 fallback
with the rather tight error bounds that are in place.