backend-vnc: use output power_state to disable repainting while disconnected
With weston_output_power_on/off()
we can use power_state
to disable repainting completely while no VNC client is connected. This allows to remove the initial repaint and per-output damage tracking.
This effectively reverts commit ce4cf2d0 ("backend-vnc: move accumulated damage with output"), which doesn't cover the case of non-moving views (dis)appearing in the moved output. A simpler solution for inconsistent damage after moving outputs could be !1208 (merged). Both are only relevant with multi-backend support (!578 (merged)).