Use Xrandr to detect screensize

DisplayHeight and DisplayWidth give the combined screen size in multimonitor and xinerama setups which causes crashing. The best solution I have found is to use Xrandr to detect allowable screen sizes. I haven't yet tested while using Xinerama, but normal multi-head no longer crashes with this patch.

edit: I don't think I can test xinerama easily with my current hardware, but its an outdated system anyways :D. I doub't it would make a difference