iiif/size: don't return zero-area rectangles
iiif/size: don't return zero-area rectangles

This will add a tiny bit of overhead since zero was previously handled
in the C libraries (openjpeg and image magick), but it ensures the final
image dimensions are going to be known ahead of time, which we have to
have in order to restrict output without the decode cost

uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server