lbximg: Fix wart in frame scanline header format

lbximg: Fix wart in frame scanline header format

The format of frame scanlines has been a little weird since day 1: We
had 2 "types" of scanline headers disambiguated by some non-trivial
logic, including whether or not it was the first decoded scanline in
the frame.

Here's a new, simpler approach, which has only one type of scanline
header, consistent meanings to the header fields, and by interpreting
the first 4 bytes as a "frame header" we get rid of the special case
for the first scanline, too.

Add a document in English which describes the LBX image file format, to
be used as the baseline format specification going forward.

Finally, update the contrived test cases for the new format.  All the
test cases using actual MOO2 data still pass.