Clean up resize filter selection issues

More careful selection of filter for resizing.

In oiiotool, previously, in cases of large magnification (zooming in by more than 2x), the entire filter could fail to cover any source pixels, resulting in black grid lines in the output.  Now we make sure that when the destination pixels are higher frequency than the source pixels, that the filter is constructed to be wide enough in "source pixel space."

I also replicated this logic into maketexture(), though I'm not sure it ever could have been symptomatic since maketx never has a need to do more than a 2x up-res (and then, only if --resize is used, which we don't recommend unless you're trying to maintain compatibility with certain non-OIIO texture systems).