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.

Change-Id: I04c9f2c828373d64e880a8c7bcade5307015ce35