libvpx

Add early termination in transform size search

In the full-rd transform size search, we go through all transform
sizes to choose the one with best rd score. In this patch, an
early termination is added to stop the search once we see that the
smaller size won't give better rd score than the larger size. Also,
the search starts from largest transform size, then goes down to
smallest size.

A speed feature tx_size_search_breakout is added, which is turned off
at speed 0, and on for other speeds. The transform size search is
turned on at speed 1.

Borg test results:
1. At speed 1,
   derf set: psnr gain: 0.618%, ssim gain: 0.377%;
   stdhd set: psnr gain: 0.594%, ssim gain: 0.162%;
   No noticeable speed change.
3. At speed 2,
   derf set: psnr loss: 0.157%, ssim loss: 0.175%;
   stdhd set: psnr loss: 0.090%, ssim loss: 0.101%;
   speed gain: ~4%.

Change-Id: I22535cd2017b5e54f2a62bb6a38231aea4268b3f