Fix flickering and performance problems with using Unredirect Fullscreen

Windows with multiple monitors.
(LP: #1050749) (LP: #1051885)

Written by smspillaz. Simplified by vanvugt.. Fixes: https://bugs.launchpad.net/bugs/1050749, https://bugs.launchpad.net/bugs/1051885. Approved by Sam Spilsbury, Daniel van Vugt.