don't draw unlock-indicator if already on screen

If we have multiple mirrored output, the unlock indicator will be drawn to both screens, and the second screen wil lshow the unlock indicator of the first, which looks bad if they don't show up at the same position. This patch tries to fix it by checking if the unlock-indicator will be visible on othe screens which already have one drawn to them.

Since i3lock has no test suite, I only tried some values for screen resolution and position, and it seems to work…