Buffers should have readlocks and writelocks to enforce
GST_BUFFER_DONTKEEP and relax the restriction that buffers with
multiple refcounts are read-only.



Example:

  videotestsrc ! ximagesink

  videotestsrc requests a buffer from its src pad

  ximagesink creates a buffer (refcount:1 readlock:0 writelock:0)

  videotestsrc writelocks it (refcount:1 readlock:0 writelock:1)

  videotestsrc writes to the buffer

  videotestsrc un-writelocks it (refcount:1 readlock:0 writelock:0)

  ximagesink readlocks it (refcount:1 readlock:1 writelock:0)

  ximagesink writes it to the screen

  ximagesink un-readlocks it (refcount:1 readlock:0 writelock:0)