Just wondering if there's a way to get sample(), when
sampling just beyond an edge pixel, to interpolate between that
edge pixel and the edge pixel on the other side of the image,
rather than with a transparent pixel ("wrap" the image).
Basically, I'm writing a simple linear-to-polar coordinates
filter, and I can see the effects of the interpolation with the
transparent pixel at around -pi to pi because those are the edges
of the transformed image.