libvpx

Re-factor bit allocation in first pass.

Restructuring to allocate the bits for each frame in
a GF group at the time the group is defined.

At the moment the allocation closely mirrors what
we had before.

Also changes the default rate adjustment method to
LONG_TERM_VBR_CORRECTION.

Change-Id: Ie5793c46c6b9c888cead5d8790792efd7d60b7c1