h264: simplify reference picture marking process.

h264: simplify reference picture marking process.

... to build the short_ref[] and long_ref[] lists from the DPB, instead
of maintaining them separately. This avoids refs/unrefs while making it
possible to generate the list based on the actual picture structure.

This also ensures that the list of generated ReferenceFrames[] actually
matches what reference frames are available in the DPB. i.e. short_ref[]
and long_ref[] entries are implied from the DPB, so there is no risk of
having "dangling" references.