Today, we track the short term RPS size for DXVA, but only if the
SliceHeader RPS is being used. Otherwise it's left uninitialized.

NVIDIA's VDPAU implementation requires that the size be accurately
tracked even if an SPS RPS is being used. In this case, it's really
counting the size of the RPS idx information, but you end up with
mangled output if the value is not accurate.

VDPAU also needs the size of the long term RPS.
Signed-off-by: Philip Langdale <philipl@overt.org>