Fix crashes with Alt-tab handling.

git-svn-id: https://www.williamfeely.info/svn/dxgl@575 8a90861a-4eca-46d5-b744-240ff16d0c4d
Force screen redraw when losing fullscreen window focus to force display out of exclusive mode.

git-svn-id: https://www.williamfeely.info/svn/dxgl@576 8a90861a-4eca-46d5-b744-240ff16d0c4d