Add superframe support for frame parallel decoding.
A superframe is a bunch of frames that bundled as one frame. It is mostly
used to combine one or more non-displayable frames and one displayable frame.
For frame parallel decoding, libvpx decoder will only support decoding one
normal frame or a super frame with superframe index.
If an application pass a superframe without superframe index or a chunk
of displayable frames without superframe index to libvpx decoder, libvpx
will not decode it in frame parallel mode. But libvpx decoder still could
decode it in serial mode.