libvpx

Add max burst bitrate control.

Applies an upper limit on burst bitrate for any
frame. This is to insure that typical encodes for YT
do not produce frames that are so large that they
risk stalling HW implementations. Such frames
could also cause playback problems in SW.

For now the limit is set at 250 bits per MB for 1080P
and larger (with the 1080P limit used for smaller frames).

Setting maxQ, constant quality mode or targeting a
very high bandwidth will have precedence over this limit.

Change-Id: Ie6f776c38b06ac7cec043d034085f4b79ee46a38