Rewrite trim_rect() for robustness.

The previous version dealt adequately with rectangles _partially_
overlapping the edge of the canvas, but doesn't correctly handle a
rectangle that's completely out of bounds in one direction. Replace
with a complete rewrite which is more easily seen to be correct. Also,
while I'm at it, add a missing condition to draw_update() so that we
don't even bother calling the Javascript half of it on any rectangle
that's been trimmed into nonexistence.

[originally from svn r9800]

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