OP noticed our one-bit xbm images demonstrated an X11 driver bug
on certain hardware. While it's good that our test programs show the bug
(e.g. test/bitmap), it's bad that our games do.
Removed the 16 xbm files used by checkers which simulated transparency
with dithered images, and replaced with 4 "hexdump" png files that are
cleaner/simpler looking for such lores pieces (46x46 pixels).
Will follow up with a separate commit for the source .png and gimp .xcf
file I used for making the pieces.
c9c51dbc48