Redraws during undo in Sixteen had been broken by my faffing about

with last_movement_sense (which was necessary to fix the animation
when the grid was only 2 squares wide in either dimension). Movement
sense is now inverted if the move being animated is an undo.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@4191 cda61777-01e9-0310-a592-d414129be87e