This commit hacks the vp9 encoder to produce bit-streams that
exercise maximum reference frame buffers at decoder, as part of
the VP9 decoder performance test suite. It fullfills all the 8
reference frame buffers first and then temporarily suspends the
buffer update. It selects the frames from this static buffer pool
as reference frames for the next a few coding frames. When all
the frames in the reference frame buffer are covered, the codec
resumes the buffer update process. Such pattern repeats every
64 frames.

It verifies the decoder capability to handle up to 8 reference
frames in buffer.

Change-Id: I796701eba53ed19ae73351d23d676311f12c43a1