Allow negative offset on x11 geometry

WxH+X+Y was supported, but -X and -Y are recognized as opposite offsets from current monitor

This allow to easily display screenkey at desired location (here at bottom right corner):

screenkey --bg-color '#fff' --font-color '#000' -g 100x100-0-0 -s large -p fixed

More info:

https://www.x.org/releases/X11R6.7.0/doc/X.7.html#sect6