Cleanups to completion flashes: all four of these games used to

Cleanups to completion flashes: all four of these games used to
redraw the whole window _every_ time game_redraw() was called during
a flash. Now they only redraw the whole window every time the
background colour actually changes. Thanks to James H for much of
the work.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@6166 cda61777-01e9-0310-a592-d414129be87e