[mutter] clutter/stage-cogl: Reduce while loop iterations

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 67a3715ded0f36fd03193adafb72e079460b7167
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Thu Jun 6 15:39:26 2019 +0800

    clutter/stage-cogl: Reduce while loop iterations
    
    If `last_presentation_time` is zero (unsupported) or just very old
    (system was idle) then we would like to avoid that triggering a large
    number of loop interations. This will get us closer to the right answer
    without iterating.
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/363

 clutter/clutter/cogl/clutter-stage-cogl.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)
---
diff --git a/clutter/clutter/cogl/clutter-stage-cogl.c b/clutter/clutter/cogl/clutter-stage-cogl.c
index 229015b90..03f067267 100644
--- a/clutter/clutter/cogl/clutter-stage-cogl.c
+++ b/clutter/clutter/cogl/clutter-stage-cogl.c
@@ -204,6 +204,19 @@ clutter_stage_cogl_schedule_update (ClutterStageWindow *stage_window,
 
   next_presentation_time = stage_cogl->last_presentation_time + refresh_interval;
 
+  /* Get next_presentation_time closer to its final value, to reduce
+   * the number of while iterations below.
+   */
+  if (next_presentation_time < now)
+    {
+      int64_t last_virtual_presentation_time = now - now % refresh_interval;
+      int64_t hardware_clock_phase =
+        stage_cogl->last_presentation_time % refresh_interval;
+
+      next_presentation_time =
+        last_virtual_presentation_time + hardware_clock_phase;
+    }
+
   while (next_presentation_time < now + min_render_time_allowed)
     next_presentation_time += refresh_interval;