fix item box detection for players 3 and 4 (Makefile typo)
change "item" representation to enum
fix threshold to avoid detecting blanks as lightning
add option for having items be part of frame comparison
make "item box with unrecognized contents" a proper enum value
sort masks at startup so positions are always correct

These changes eliminate a lot of frames that really should have been marked
"unknown" because the item box was in transition.  It also fixes detection of
some others.  All of the frames I've spot-checked match valid item masks, and
nearly all of them are totally clear.

The sorting change could in principle have a significant effect on other,
non-item mask detection.  (It may actually fix several bugs.)