[FIXES #7] fixes mysterious crasher.

When scaling using aspect fill, if an image needed to be offset in order to center on the x-axis (because the image is taller than wide) there was occasionally a crash deep in memcpy. This happens because of the way a CGImageContext creates an image whose width doesn't correspond to its rowBytes—a buffer overrun can happen. By reducing the height by 1-pixel in this situation, we avoid the buffer overrun.
Merge pull request #21 from origamilabs/fix-7

[FIXES #7] fixes mysterious crasher.