libvpx

Add experimental VBR adaptation method.

Add code to monitor over and under spend and
apply limited correction to the data rate of subsequent
frames. To prevent the problem of starvation or overspend
on individual frames (especially near the end of a clip) the
maximum adjustment on a single frame is limited to a %
of its un-modified allocation.

Change-Id: I6e1ca035ab8afb0c98eac4392115d0752d9cbd7f