Teach WOHUDScroller to draw arrows in the scroller buttons (REnamer, f3b70d5)

There is still some scope for refactoring here; note that the up arrow in the flipped coordinate system is the same as the down arrow in the unflipped coordinate system, and the flipped down arrow is the same as the unflipped up arrow.

Signed-off-by: Greg Hurrell <greg@hurrell.net>