vnc: Fix cursor updates

Now that overlapping outputs are a thing, we have a problem with vnc
cursors.

The surface->damage used to update the vnc cursor might actually be
flushed by a previous output's repaint cycle, leading to a missing cursor
update to the vnc client.

Instead we should use the damage accumulated on the cursor plane to choose
when to update the cursor. This damage is in output coordinates, so let's
be lazy and just use the presence of damage as an indicator that the
cursor needs an update.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>