Swap alt/gold/new/last frame buffer ptrs instead of copying.

At the end of the decode, frame buffers were being copied.
The frames are not updated after the copy, they are just
for reference on later frames.  This change allows multiple
references to the same frame buffer instead of copying it.

Changes needed to be made to the encoder to handle this.  The
encoder is still doing frame buffer copies in similar places
where pointer reference could be done.

Change-Id: I7c38be4d23979cc49b5f17241ca3a78703803e66