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