Fix output of idiff and oiiotool --diff when channels differ

Fix output of idiff and oiiotool --diff, they could crash when printing
in cases where the two images don't have the same number of channels.
And for oiiotool, make it like idiff where it can compare images that
have different number of channels or pixel windows (in non-overlapping
areas, the values of the missing pixels or channels are presumed 0).