Bring back the clamping, but instead explicitly query the root window size...

Bring back the clamping, but instead explicitly query the root window size before building monitor structs.

The issue was that DisplayWidth()/DisplayHeight() values don't get
updated after XOpenDisplay(); the fix is querying the root window size
instead to update the auth window size.

The clamping is there to handle the (hopefully unlikely) case of a
screen being only partially in the root window (i.e. XRandR vs X11 core
inconsistency); it's no longer harmful once the bug is fixed.