Add a global mutex to exclusive access to shared stream buffers, such
as DMABufs or VASurfaces after a tee:

LIBVA_DRIVER_NAME=iHD \
gst-launch-1.0 v4l2src ! tee name=t t. ! queue ! \
  vapostproc skin-tone=9 ! xvimagesink \
  t. ! queue ! vapostproc ! xvimagesink

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