Fix for 'stuck in full screen' issue

When GC is restarted after being closed full screen, the saved geometry
is larger than the actual screen size - resulting in the title bar being
drawn off screen & not easily reachable.

This patch checks whether the saved geometry is larger than the current
screen size, and resizes to the screen size instead of the saved values.
Merge pull request #532 from dresco/geometry-fix

Fix for 'stuck in full screen' issue