Let scaling-aware Xwayland clients scale themselves with "scale-monitor-framebuffers"

Based on this branch from Jonas Ådahl, main commit:

Apply custom scaling to Xwayland
    
When monitor framebuffers are scaled, this special cases Xwayland and
sends output regions in a way that Xwayland think everything is N times
as large as the logical region, where N is the ceil of the max monitor
scale.
    
This is done by introducing a "stage" vs "protocol" coordinate space for
X11, where the "protocol" coordinate space is "stage" multiplied by a
scaling factor.

Goes with gnome-settings-daemon!353 (merged)