downscale thumbnail if requested size is smaller than it is, do not hardcode 128
workaround for (a bug in?) libopenraw:
if you request size bigger than 160 (the exif thumbnail), but smaller than 362 -
it returns a garbled 362x234 image, at least in case of 5D mark2 CR2 files,
so let's skip this range. 512 returns the big thumbnail (full or quarter size)