videoaggregator: Changing playback rate is broken

Submitted by Thibault Saunier @thiblahute

Link to original bug (#775102)

Description

Validate tests: validate.file.glvideomixer.simple.fast_forward.synchronized
Result: HARD timeout, last more than 150 secs (this test should last ~14 seconds (which is the case with compositor))

Failure example: https://ci.gstreamer.net/job/GStreamer-master-meson-validate/268/testReport/junit/validate.file.glvideomixer.simple/fast_forward/synchronized/

Pipeline:

GST_VALIDATE_SCENARIO=fast_forward GST_GL_XINITTHREADS=1 \
gst-validate-1.0 glvideomixer sink_1::alpha=0.5 sink_1::xpos=50 sink_1::ypos=50 name=_mixer ! deinterlace ! videoconvert ! autovideosink videotestsrc pattern=snow timestamp-offset=3000000000 ! 'video/x-raw,format=AYUV,width=640,height=480,framerate=(fraction)30/1' ! timeoverlay ! _mixer. videotestsrc pattern=smpte ! 'video/x-raw,format=AYUV,width=800,height=600,framerate=(fraction)10/1' ! timeoverlay ! _mixer.

I have not investigated but we can see that the pipeline plays very smoothly with compositor but it is very jerky with glvideomixer.