Previously, global output offset only moved outputs if they were in
negative coordinate space. Doing so guaranteed an output on the top-most
and left-most border of the screen. This commit offsets outputs to achieve
the same goal on outputs in positive coordinate space, which matches the
output placement of `xrandr`, a placement assumption made by Krita's
tablet tool logic.
The new definition of global output offset can be thought of as drawing
the smallest possible rectangle with contains every monitor, and moving
that bounding box to (0, 0).
