Which finally allows us to use the PLANE_SIZE macro in ASM land. Yeah,
(ROW_SIZE * RES_Y) has finally got old.
Part of P0073, funded by [Anonymous] and -Tom-.
"bgimage" really was the best name I could come up with here, given
what it's used for. Everything else sounded way too ambiguous. Even
something like "mempage", in the meaning of "a third memory-backed
graphics page"… --Nmlgc