Improve the partition search breakout speed feature

Improve the partition search breakout speed feature

Use a linear model to make partition search breakout decisions.
Currently the model is tuned for large quantizers and small resolutions.
So it is only used when q-index is larger than 200 and frame
width/height is smaller than 720. Also it's not yet supported for high
bit depth.

Tested speed 1 and 2 on lowres and midres. Compression performance is
neutral. At low bitrates, encoding speedup is up to 50% for speed 1;
up to 30% for speed 2.
Some sample numbers:

into_tree_480p, speed 1
QP=60 before:  35.228 dB, 3488b/f, 7.78 fps
      now:     35.217 dB, 3475b/f, 11.57 fps
QP=50 before:  37.492 dB, 7983b/f, 6.24 fps
      now:     37.491 dB, 7974b/f, 7.55 fps

PartyScene_832x480_50, speed 1
QP=60 before:  30.104 dB, 22426b/f, 3.28 fps
      now:     30.109 dB, 22410b/f, 4.43 fps
QP=50 before:  33.016 dB, 46984b/f, 2.78 fps
      now:     33.018 dB, 46998b/f, 3.35 fps

into_tree_480p, speed 2
QP=60 before:  35.175 dB, 3506b/f, 10.96 fps
      now:     35.185 dB, 3510b/f, 13.47 fps
QP=50 before:  37.448 dB, 8016b/f, 9.04 fps
      now:     37.459 dB, 8048b/f, 9.81 fps

PartyScene_832x480_50, speed 2
QP=60 before:  30.060 dB, 22537b/f, 4.42 fps
      now:     30.061 dB, 22541b/f, 5.38 fps
QP=50 before:  32.923 dB, 47134b/f, 3.85 fps
      now:     32.920 dB, 47073b/f, 4.31 fps

Change-Id: I674cba4f027c4c65f7837d5ec9179d6201e6ba86