coloring in the contour window to make only a single call to
invalidate-bitmap-cache
It was making two before, which could have pretty similar regions,
leading to two repaints instead of just one. When pushing the down
arrow near the bottom of the visible regions of the definitions text
was causing those two calls to take about 30 milliseconds and after
this change the one call seems to be about half that (but there is
lots of variance so I'm only sure that it helped some, not how much)
original commit: 6fc70a04d8b69f41d82c3bcbeeff26254017052d
1a5c4047dc