We now properly handle OS/2 format BMPs that use 3 bytes per color
entry instead of 4. While OS/2 2.x officially specified 4 bytes per
color, some tools still produce files with 3-byte entries. We can
identify such files by checking the available color table space.
Color masks should only be used when the compression type is either
BITFIELDS or ALPHABITFIELDS. They were always read before and produced
corrupted images when there was random data in the mask fields.
Other browsers don't think that BMP files with more than 1024 colors are
invalid. They clamp the palette instead, and now we do the same. This
allows us to load more BMPs.
