backend-pipewire: add fence to synchronize on finish of render

If the PipeWire-backend renders to a DmaBuf, try to use a fence to synchronize
the submit of the next PipeWire buffer to the completion of the current render
process.

This ensures that the GPU rendering is finished before the buffer is passed to
PipeWire.

If fences are not available, the buffer is submitted without explicit
synchronization as before.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>