Support geometry in --bg-max and --bg-center

Overloads the --geometry option for wallpaper setting
Currently only supports the two options that result in
black borders, --bg-max and --bg-center.

For both, if a geometry offset is specified (w/h is ignored),
the image will be offset the specified pixels from the screen edge
Left side for + numbers, right side for -.
+/-0 will align at the edge of the screen.