Earlier, clearer error when font can't be found at all
oiiotool --over and --zover should set output display window to union, not fg

I hadn't noticed a problem before because fg and bg are usually the same
size. But if they differ, it seems more likely that it's a small element
being put overtop a fixed background, so setting the output's display
("full") window to the fg is not what you want.
More portable definition of stride_t and AutoStride for array_view