Fixes to oiiotool --fit and IBA::resize

Production users noticed some flaws with oiiotool --fit.

1. Images where the data and display windows were not the same (i.e. the pixel data had a nonzero offset) did not "fit" properly.  This necessitated an overhaul to the "fit" logic, now it works much better, even with offsets, for both "portrait" and "landscape" images, and even if the aspect ratio flips.

2. There could be artifacts in the resize step that stemmed from our practice of "clamping" the position to the image data when the resize filter extended past the pixel data window. A change in logic to simply ignore (not contribute to the weighted sum) the part of the filter past the image edge gives pleasant results without the artifacts, and simplifies the code in the function as well.