resample: improve performance (thanks José Miguel Sánchez García)

Resample is well known for taking a long time to resize an image. This
patch brings an important performance boost (in my test image, time
was reduced from ~2850ms to ~500ms). It does that by extracting FP
multiplication and division out of the innermost loop of
resamplex/resampley.

The results differ slightly from the current implementation: in my
test: ~0.3% of the bytes had a ±2 difference in their value, which I
attribute to rounding errors. I'm personally not concerned with that
deviation, given the performance gains. However, I recommend testing
it just to be sure I didn't overlook anything.

José Miguel Sánchez García

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