Synchronization precision

[RiccardoCagnasso]

I took some time to read your answer, the discussion you linked and some of
the code in your branch. I think I managed to understand what you are trying
to do and what problems you are facing, at least to an extent (which is an
achievement by itself). I'm not sure that I can give any meaningful insight
about that though. Except for maybe the fact that synchronizing the
gstreamer clock with the wayland presentation layer that way doesn't sound
right to me. It feels like the process of synchronizing with vblanks shoud
be confined to the video sink and not have a larger impact on the pipeline
(and clock) as a whole. But this is probably a stupid observation since I
don't really know much about gstreamer internals, so meh.

What I also did, possibly less pointless, is to make some tests with
glimagesink. I'll link some results here because they are puzzling.

1) https://youtu.be/RFRUZrsllnU
This is my laptop, with an Intel video card, vaapidecode/vaapisink and both
videos on the same monitor. You can see random desyncs consistent with what
we discussed.

2) https://youtu.be/C2lDp-Lh8B8
This is one of my workstations with a Quadro P400. Vsync is enabled and full
composition pipeline. nvdec and glimagesink. Both videos on one monitor. The
sync is perfect as far as I can see.

3) https://youtu.be/wwvfbHGh4a0
This is the same workstation but with one video on each monitor. One video
seems exactly one frame later than the second. 



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