Add mode info arrays and mode info index.
In non frame-parallel decoding, this works the same way as current decoding scheme. Every time after decoder finish decoding a frame, it will swap the current mode info pointer and previous mode info pointer if the decoded frame needs to be shown. Both mode info pointer and previous mode info pointer are from mode info arrays. In frame-parallel decoding, this will become more complicated as current frame's mode info pointer will be shared with next frame as previous mode info pointer. But when one decoder thread finishes decoding one frame and starts to work on next available frame, it needs to retain the decoded frame's mode info pointers until next frame finishes decoding. The mode info index will serve this purpose. The decoder will use different buffer in the mode info arrays and use the other buffer to save previous decoded frame’s mode info. Change-Id: If11d57d8eb0ee38c8876158e5482177fcb229428