Implement new handling of the LBX image header data.

Implement new handling of the LBX image header data.

* Change "#offsets" to "#frames".  This makes lbximg capable of decoding all
  frames of images such as the TANM_ family and science.lbx.001
* Change interpretation of "pal" to a bitmask containing flags.  Current flags
  handled are
   - 0x0400: render each frame on a blank slate (makes TANM_ family work).
   - 0x1000: image contains embedded palette, as previously understood.
   - 0x2000: image loops over all frames.
* Change interpretation of previously-thought "#frames" to "leadin" - that is,
  indicates the first frame that is part of a looping animation.  This is
  superseded by flag 0x2000, and yes - leadin == 0 means the same thing as that
  flag.  This will allow science.lbx.001 to be looped correctly.  This leads to
  the somewhat strange notion that all images loop, just that some simply loop
  over the last frame.

Also update lbx_getinfo() to reflect the new information.