Include code in midend.c to sanitise streams of mouse events so that

back ends can be assured of always receiving them in a sensible
sequence (button-down, followed by zero or more drags, followed by
button-up, and never changing button in the middle of such a
sequence). I have a suspicion this issue was the cause of the
mysterious Pattern grid updates seen by Dan during testing last
night.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5208 cda61777-01e9-0310-a592-d414129be87e