Revert the optimization of the bilinear weights.

Revert the optimization of the bilinear weights.

For the case where the resampling changed every frame (e.g. a zoom),
it just consumed too much CPU to be worth it, especially in memory
management; this is painful because it was an elegant solution to
a tricky problem, but it just has to go for now.

Also drop out to fp32 at the first sight of too-high error.