[mutter] clutter/frame-clock: Comment presentation time skip

[Marge Bot <marge-bot src gnome org>]

commit 01b30ff9e4406eb2c48af3f145e08990e3dcaa55
Author: Ivan Molodetskikh <yalterz gmail com>
Date:   Fri Nov 27 18:16:33 2020 +0300

    clutter/frame-clock: Comment presentation time skip
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1707>

 clutter/clutter/clutter-frame-clock.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)
---
diff --git a/clutter/clutter/clutter-frame-clock.c b/clutter/clutter/clutter-frame-clock.c
index 6a32ea3d7c..2f617d0a59 100644
--- a/clutter/clutter/clutter-frame-clock.c
+++ b/clutter/clutter/clutter-frame-clock.c
@@ -280,7 +280,20 @@ calculate_next_update_time_us (ClutterFrameClock *frame_clock,
       next_presentation_time_us = logical_clock_phase_us + hw_clock_offset_us;
     }
 
-  /* Skip one interval if we got an early presented event. */
+  /*
+   * Skip one interval if we got an early presented event.
+   *
+   *        last frame this was last_presentation_time
+   *       /       frame_clock->next_presentation_time_us
+   *      /       /
+   * |---|-o-----|-x----->
+   *       |       \
+   *       \        next_presentation_time_us is thus right after the last one
+   *        but got an unexpected early presentation
+   *             \_/
+   *             time_since_last_next_presentation_time_us
+   *
+   */
   last_next_presentation_time_us = frame_clock->next_presentation_time_us;
   time_since_last_next_presentation_time_us =
       next_presentation_time_us - last_next_presentation_time_us;