compositor: Respect repaint-window when restarting repaint loop.

If a stopped repaint loop gets restarted due to posting of new
damage, and this restart of the repaint loop happens late in the
video refresh cycle, ie. already inside the repaint-window and
thereby after the composition deadline for the current frame,
then defer the actual output repaint to the composition deadline
of the next video refresh cycle by setting the repaint timer
accordingly.

This tries to make sure that:

a) Client(s) posting damage timely before the composition deadline
   (video refresh duration - "repaint-window" duration) of the
   current refresh cycle will trigger a repaint within the current
   refresh cycle, thereby avoiding one extra frame of compositor
   lag due to the needed restart of the repaint loop if the loop
   was stopped. This allows them to benefit from the earlier
   "instant repaint restart" commit to keep latency low.

b) Late clients which post damage close to the end of a refresh
   cycle can't race other clients if the repaint loop is restarted.
   Instead they will get deferred to the next compositor cycle,
   just as if the repaint loop would have been already running -
   the semantic of the "repaint-window" parameter is preserved.

   This is especially important to prevent a very late client
   from triggering a repaint very close to the vblank, which
   would cause the compositor to certainly miss the vblank and
   skip one frame and then cause a delay of another frame for
   other clients which posted their damage in time for the
   following frame. Iow. this provides clients with a more
   predictable compositor timing and makes it easier for them
   to latch onto the compositors repaint cycle.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/compositor.c

@@ -2373,6 +2373,13 @@ weston_output_finish_frame(struct weston_output *output,
 		msec = 0;
 	}
 
+	/* Called from restart_repaint_loop and restart happens already after
+	 * the deadline given by repaint_msec? In that case we delay until
+	 * the deadline of the next frame, to give clients a more predictable
+	 * timing of the repaint cycle to lock on. */
+	if (presented_flags == PRESENTATION_FEEDBACK_INVALID && msec < 0)
+		msec += refresh_nsec / 1000000;
+
 	if (msec < 1)
 		output_repaint_timer_handler(output);
 	else