This patch makes further changes to support an experimental
corpus wide VBR mode that uses a corpus complexity
number as the midpoint of the distribution used to allocate bits
within a clip, rather than some average error score derived from the
clip itself.

At the moment the midpoint number is hard wired for testing and
the mode is enabled or disabled through a #ifdef.  Ultimately this
would need to be controlled by command line parameters.

Change-Id: I9383b76ac9fc646eb35a5d2c5b7d8bc645bfa873