Add support for offsets in ResampleEffect.

Add support for offsets in ResampleEffect.

This enables smooth (subpixel) panning that people frequently want for stills
and titles, but that you couldn't do in a subpixel fashion before (PaddingEffect
could only do integer pixel offsets).

The placement (ResampleEffect) might seem a bit off at first, but subpixel
offset needs resampling, and ResampleEffect already has all the logic in place
for that. We could have used the GPU's built-in bilinear resampling, of course,
but it doesn't look all that good for high-contrast situations (although working
in linear light should help some).