i3bar: Fix pixel offset calculation in dual-head

i3bar: Fix pixel offset calculation in dual-head

When running i3bar on a dual-head display with workspaces enabled in i3bar's
config, the second monitor wouldn't use a correct x position offset and
would draw its label too close to the screen's edge.  In comparison to the
first monitor, this is inconsistent because that starts with a pixel offset
of 1.