liblbx: Add support for "chunked" images.

It appears that the frame count fields aren't actually two 16-bit
fields, but rather four 8-bit fields.  The images in monster.lbx
appear to have "chunks" of animation and what was previously thought to
be the most significant byte of the lead-in now seems to be the length
of a "chunk".

These images are much like any other, except that the first frame of
every chunk is drawn on a clean slate.  Examples are found in
monster.lbx.