[mutter] clutter/frame-clock: Advance timelines according to presentation time

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

commit 7e3d1e26a15b6ddd9a49d4e42718f125b57f4394
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Tue Dec 14 17:49:59 2021 +0800

    clutter/frame-clock: Advance timelines according to presentation time
    
    This ensures they remain perfectly smooth regardless of how the
    dispatch time has been adjusted/optimized/delayed/jittered.
    
    Idea by Ivan Molodetskikh <yalterz gmail com>
    
    For example, dragging a window on a 60Hz monitor:
    
    BEFORE
    
    delta(time_us) = 17014μs
    delta(time_us) = 15998μs
    delta(time_us) = 17006μs
    delta(time_us) = 16975μs
    delta(time_us) = 16001μs
    delta(time_us) = 17002μs
    delta(time_us) = 17006μs
    delta(time_us) = 16004μs
    
    AFTER
    
    delta(time_us) = 16667μs
    delta(time_us) = 16667μs
    delta(time_us) = 16670μs
    delta(time_us) = 16667μs
    delta(time_us) = 16669μs
    delta(time_us) = 16668μs
    delta(time_us) = 16664μs
    delta(time_us) = 16674μs
    
    Caveat 1: Because we don't know a "next presentation time" on the first
    frame, the interval between the first and second frame will usually be
    different to the subsequent steady interval. So this change increases the
    jitter of just frame 2, but eliminates jitter thereafter.
    
    Caveat 2: `clutter_frame_clock_schedule_update_now` schedules updates
    earlier than `clutter_frame_clock_schedule_update`. This means potentially
    you could get multiple frames targeting the same "next presentation time".
    That doesn't really change here though - we're dispatching at the same
    times as we used to and just giving timelines a better vsync-aligned
    timestamp now.
    
    Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/25
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2161>

 clutter/clutter/clutter-frame-clock.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/clutter/clutter/clutter-frame-clock.c b/clutter/clutter/clutter-frame-clock.c
index 24e0ec8a22..9be9898ee1 100644
--- a/clutter/clutter/clutter-frame-clock.c
+++ b/clutter/clutter/clutter-frame-clock.c
@@ -642,6 +642,8 @@ clutter_frame_clock_dispatch (ClutterFrameClock *frame_clock,
   COGL_TRACE_END (ClutterFrameClockEvents);
 
   COGL_TRACE_BEGIN (ClutterFrameClockTimelines, "Frame Clock (timelines)");
+  if (frame_clock->is_next_presentation_time_valid)
+    time_us = frame_clock->next_presentation_time_us;
   advance_timelines (frame_clock, time_us);
   COGL_TRACE_END (ClutterFrameClockTimelines);