Account for screen offset when calculating clickable area positions

Works with XINERAMA now. Tested with:
echo '\ableft1\ac echo left1 \ae\c\abcenter1\ac echo center1 \ae\r\abright1\ac echo right1 \ae\sn\ableft2\ac echo left2 \ae\c\abcenter2\ac echo center2 \ae\r\abright2\ac echo right2 \ae' | ./bar -p