Fix crash when struts change during grab operation

Since meta_workspace_invalidate_work_area() frees the edges
workspace->screen_edges and workspace->monitor_edges, we must clean up
our cached edge resistance data when the invalidate_work_area() is
called on the active workspace, or when the workspace changes.

Make the computation of the edge resistance data lazy so that it
will be recomputed the next time we try to access it.
meta_display_compute_resistance_and_snapping_edges() is made
private to edge-resistance.c

Invaliding the data when active workspace changes also will improve
correctness for edge resistance when the current workspace changes
during a grab operation. (Even with this fix we still don't try to
handle window positions changing during a grab operation; that can't
cause a crash since, unlike screen and monitor edges, the window edges
are freshly allocated, it will just cause slight oddness in that
corner case.)

Root cause tracked down due to much effort by Jon Nettleton.
https://bugzilla.gnome.org/show_bug.cgi?id=608800 (Mutter)
https://bugzilla.gnome.org/show_bug.cgi?id=603632 (Metacity)

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