[Bug 781928] New: videoaggregator: issues with live mode when the input framerates are different from the output framerate

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=781928

            Bug ID: 781928
           Summary: videoaggregator: issues with live mode when the input
                    framerates are different from the output framerate
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-plugins-bad
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: mduponchelle1 at gmail.com
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

How to reproduce:

Get wondershaper (available on Fedora at least).


```
sudo wondershaper wlp6s0 1000000 100
GST_DEBUG=*videoaggregator*:6 gst-launch-1.0 videotestsrc pattern=ball
is-live=true ! videorate ! video/x-raw, framerate=30/1 ! identity ! compositor
name=comp sink_0::alpha=0.5 sink_1::alpha=0.5 sink_2::alpha=0.5 ! video/x-raw,
framerate=5/1 ! queue ! xvimagesink  uridecodebin
uri=https://media.w3.org/2010/05/sintel/trailer.webm ! videorate ! comp. 2>&1 |
grep "Producing buffer\|replacing old buffer\|Taking new buffer"
```

Observed behaviour:

The ball moves at 5fps as expected, but its movement is 6 times slower than it
would otherwise be.

The timestamps that are mixed progressively drift from the output timestamps.

Expected behaviour:

The ball should move at 5fps, but 5 out of 6 frames should be skipped, thus
maintaining the speed of its movement.

Of course this is a simple example to highlight the problem, with actual
sources the decoder very quickly ends up dropping frames because of QOS.

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