libvpx

Modified Handling of min and max vbr rates.

In two pass encodes bits are allocated to each frame
according to a modified error score for the frame as a
fraction of the modified error score for the clip or section.

Previously a minimum rate per frame was reserved and
subtracted from the bits allocatable by the two pass code.
The vbr max section rate was enforced by clipping the
actual number of bits allocated.

In this patch the min and max vbr rates are enforced
instead by clipping the modified error scores for each frame
rather than the number of bits allocated.

Small gains for all test sets (psnr and SSIM) ranging from
~ +0.05 for YT psnr up to ~ +0.25 for Std-hd SSIM.

Change-Id: Iae27d70bdd3944e3f0cceaf225bad2e8802833de