Use the bilinear sampling trick in ResampleEffect.

Use the bilinear sampling trick in ResampleEffect.

This is a bit more complex than in BlurEffect, since we can have negative weights.
I had to adjust the test that's based on canonical output; I'm actually not 100%
sure, but everything else passes, the normal output looks fine, and who knows if
the previous value was actually the more accurate anyway (the difference is
just barely large enough to trigger the test).

Increases speed by over 50% on my machine.