liblbx: Add support for "raw" LBX images.

There is a new image format in town.  If a particular flag bit is set,
then there are no row headers and data for every pixel of a frame is
simply stored in row-major order.  An example of such an image is
starbg.lbx.009, as well as several others in the same archive.

Don't you just *love* the designers of this format?