Use rate/distortion thresholds to control non-RD partition search

Compare the estimated rate and distortion to the thresholds scaled
according to the operating block size and determine if further
split partition search will be run. The compression performance of
speed -5 is changed by -0.074%. The encoding speed is 10% - 15%
faster.

vidyo1 720p
16545 b/f, 40.492 dB, 11475 ms -> 16535 b/f, 40.486 dB, 10100 ms

nik720p
16624 b/f, 36.310 dB, 10071 ms -> 16617 b/f, 36.313 dB, 8346 ms

Change-Id: Ic9197ab5761279ae55d2fb7813b2af0e0db497b8
Remove unused VAR_BASED_FIXED_PARTITION flag

Change-Id: I4ce19b7cb1c45fed86e81ee785e787630020fb4f
Add highbitdepth function for vp9_avg_8x8

Cherry-picked from https://gerrit.chromium.org/gerrit/#/c/71914/
(a92f987a6b7819ae5c62a429e126e1c26bdb1b71) on highbitdepth branch.

Change-Id: I6903e4e4cb57d90590725c8a1c64c23da7ae65e8
Merge "Use rate/distortion thresholds to control non-RD partition search"
Merge "Remove unused VAR_BASED_FIXED_PARTITION flag"
Merge "Add highbitdepth function for vp9_avg_8x8"