compositor: warn about insane repaint delay

Make the sanity check more explicit and log a warning if it happens.

Small negative values are ok because it just means the compositor is
lagging behind, or more likely the user specified a too long repaint
window.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-By: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Bryce Harrington <b.harrington@samsung.com>

src/compositor.c

@@ -2412,8 +2412,18 @@ weston_output_finish_frame(struct weston_output *output,
 	msec = (refresh_nsec - timespec_to_nsec(&gone)) / 1000000; /* floor */
 	msec -= compositor->repaint_msec;
 
-	/* Also sanity check. */
-	if (msec < 1 || msec > 1000)
+	if (msec < -1000 || msec > 1000) {
+		static bool warned;
+
+		if (!warned)
+			weston_log("Warning: computed repaint delay is "
+				   "insane: %d msec\n", msec);
+		warned = true;
+
+		msec = 0;
+	}
+
+	if (msec < 1)
 		output_repaint_timer_handler(output);
 	else
 		wl_event_source_timer_update(output->repaint_timer, msec);