background: Scale monitor_area after texture creation
monitor_area before texture creation was just wasting
megabytes of memory on resolution that the monitor can't display. This
was also hurting runtime performance.
Monitor is natively 1920x1080 and scale set to 3.
Before: The monitor texture allocated was 5760x3250x4 = 74.6 MB
After: The monitor texture allocated is 1920x1080x4 = 8.3 MB
Closes: gnome-shell#2118 (closed)