My background-erasing changes in r5996/r5997 caused trouble with the

My background-erasing changes in r5996/r5997 caused trouble with the
keyboard-control cursors, and when I tried to fix those more
wallpaper bubbles popped up elsewhere. Here's what I think is a
proper fix: a comprehensive overhaul of the redraw code such that,
instead of tracking the cursor positions explicitly in the
drawstate, we instead track for each peg position whether or not a
cursor is currently displayed at that position. So cursor erasing
and cursor drawing become part of the main draw loop rather than a
separate bit on the end.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@6006 cda61777-01e9-0310-a592-d414129be87e