Apply machine learning-based early termination in VP9 partition search

Apply machine learning-based early termination in VP9 partition search

This patch was based on Yang Xian's intern project code. Further modifications
were done.
1. Moved machine-learning related parameters into the context structure.
2. Corrected the calculation of sum_eobs.
3. Removed unused parameters and calculations.
4. Made it work with multiple tiles.
5. Added a speed feature for the machine-learning based partition search
early termination.
6. Re-organized the code.

The patch was rebased to the top-of-tree.

Borg test BDRATE result:
4k set:     PSNR: +0.144%; SSIM: +0.043%;
hdres set:  PSNR: +0.149%; SSIM: +0.269%;
midres set: PSNR: +0.127%; SSIM: +0.257%;

Average speed gain result:
4k clips: 22%;
hd clips: 23%;
midres clips: 15%.

Change-Id: I0220e93a8277e6a7ea4b2c34b605966e3b1584ac