This is actually used for images without Rgn as well. The new code
should be easier to understand, and it handles all of the black and
white images from the Mac versions of Loom, Indy 3, Indy 4, MI1, and
MI2. It's still unfortunate - and perhaps unnecessary - that we had two
different decoders for Rect/Rgn, but I'm not comfortable modifying the
old one. Maybe later.