Fix nasty Selection + Crop + Undo bugs

This has been plaguing me for awhile, but I think I've finally arrived
at a consistent solution.  Undoing certain selection+crop combinations
should no longer crash PD.  (Fairly important, no?)

There are still some oddities with Undoing a long chain of selection
actions, where sometimes, the selection will inexplicably not appear on
the screen.  Clicking Undo/Redo causes it to reappear, so the selection
data seems to be intact, but somehow it's not activating properly.  I'm
still working on tracking down that bug, but at least it doesn't crash
the program...