Whew, time to look at every `int` variable we ever declared! The best
moment to do this would have been a year ago, but well, better late
than never. No need to communicate that in comments anymore.
These shouldn't be used for widths, heights, or sprite-space
coordinates. Maybe we'll cover that another time, this commit is
already large enough.
Part of P0111, funded by [Anonymous] and Blue Bolt.
"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