libvpx

Improves constant qual, constrained qual turned on

Adds modeled functions to decide the qp for altref frames in constant q
mode similar to other functions in use in bitrate mode.

Also turns on the constrained quality mode (end-usage=2) option which
was turned off before. Basic testing shows the mode works in principle,
to cap bitrate to the target-bitrate specified, while allowing lower
bitrate depending on the cq-level specified. The mode will need to be
improved over time.

Results for constant quality vs bitrate control mode:
derfraw300/fullderfraw: +3.0% at constant quality over bitrate control.
fullstdhdraw: +4.341%
stdhdraw250: +5.361%

Change-Id: If5027c9ec66c8e88d33e47062c6cb84a07b1cda9