Use `--clip xinerama` instead of manually calculating clip offsets

[creshal]

Use '-clip xinerama0' to clip to the first xinerama
sub-screen (if xinerama is active).  xinerama1 for the
2nd sub-screen, etc.  This way you don't need to figure
out the WxH+X+Y of the desired xinerama sub-screen.
screens are sorted in increasing distance from the
(0,0) origin (I.e. not the Xserver's order).

Ought to be xinerama1 for the common use case of no other screen being connected (and easily adjusted for other setups), which makes the whole script a lot more more robust.