Log Message:
Update to 8692:

Single mouse clicks to toggle individual grid edges stopped working
in r8414 as a side effect of reorganisation in interpret_move().
Reinstate them.

Add a couple of missing checks in validate_desc(), without which
bogus game IDs were getting as far as new_game() and failing
assertions.

Minor rewording and typo-correction.

Aha, I've managed to prove that my inadequate error highlighting is
actually just about adequate after all. Large comment added
containing some discussion and the proof.

Error-highlighted trees look nicer with a different-coloured trunk.
Also added a comment worrying about the universality of my error
highlighting mechanism.

Redo Mines and Inertia's mine graphics using an actual circle rather
than an approximating octagon, to improve the look when zoomed to
high resolution.

Tweak to the promptness of error highlighting display.

About time I got round to this: error highlighting for Tents.

Fix width/height braino introduced in r5844.