Add zooming to ResampleEffect.

Same rationale as with the offset; we need resampling for proper zoom.

The look at heavy zoom isn't _quite_ what I had hoped for (although it's OK),
and there's a hint of shimmering in the zoom center if there's high-contrast
material there. For now, I'll write off the latter as Lanczos ringing;
I'll need to see what it does to video eventually (only tested with stills).