v4l2sink: don't render preroll buffers

v4l2sink: don't render preroll buffers

Most v4l2 drivers will get upset when you queue the same buffer twice in a
row without first dequeueing it.

Rendering of pre-roll buffers can be re-introduced later, but will require
tracking the state of the buffer, and avoiding to re-QBUF if the buffer has
already been passed to the driver.