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