libvpx

Early termination in encoding partition search

In the partition search, the encoder checks all possible
partitionings in the superblock's partition search tree.
This patch proposed a set of criteria for partition search
early termination, which effectively decided whether or
not to terminate the search in current branch based on the
"skippable" result of the quantized transform coefficients.
The "skippable" information was gathered during the
partition mode search, and no overhead calculations were
introduced.

This patch gives significant encoding speed gains without
sacrificing the quality.

Borg test results:
1. At speed 1,
   stdhd set: psnr: +0.074%, ssim: +0.093%;
   derf set:  psnr: -0.024%, ssim: +0.011%;
2. At speed 2,
   stdhd set: psnr: +0.033%, ssim: +0.100%;
   derf set:  psnr: -0.062%, ssim: +0.003%;
3. At speed 3,
   stdhd set: psnr: +0.060%, ssim: +0.190%;
   derf set:  psnr: -0.064%, ssim: -0.002%;
4. At speed 4,
   stdhd set: psnr: +0.070%, ssim: +0.143%;
   derf set:  psnr: -0.104%, ssim: +0.039%;

The speedup ranges from several percent to 60+%.
                 speed1    speed2    speed3    speed4
(1080p, 100f):
old_town_cross:  48.2%     23.9%     20.8%     16.5%
park_joy:        11.4%     17.8%     29.4%     18.2%
pedestrian_area: 10.7%      4.0%      4.2%      2.4%
(720p, 200f):
mobcal:          68.1%     36.3%     34.4%     17.7%
parkrun:         15.8%     24.2%     37.1%     16.8%
shields:         45.1%     32.8%     30.1%      9.6%
(cif, 300f)
bus:              3.7%     10.4%     14.0%      7.9%
deadline:        13.6%     14.8%     12.6%     10.9%
mobile:           5.3%     11.5%     14.7%     10.7%

Change-Id: I246c38fb952ad762ce5e365711235b605f470a66