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vp9: Nonrd variance partition: increase threshold for using 4x4 avg.

In variance partition low resolutions may use varianace based on
4x4 average for better partitioning.
Increase the threshold for doing this at speed = 8.

Improves speed by ~5%, with little loss, < 1%, on RTC_derf set.

Change-Id: Ib5ec420832ccff887a06cb5e1d2c73199b093941

--- a/vp9/encoder/vp9_encodeframe.c

+++ b/vp9/encoder/vp9_encodeframe.c

@@ -799,6 +799,7 @@

   int min_var_32x32 = INT_MAX;

   int var_32x32;

   int avg_16x16[4];

+  int64_t threshold_4x4avg;

   NOISE_LEVEL noise_level = kLow;

   uint8_t *s;

   const uint8_t *d;

@@ -833,6 +834,9 @@

     }

   }

+  threshold_4x4avg =

+      (cpi->oxcf.speed < 8) ? thresholds[1] << 1 : thresholds[2] >> 1;

+

   memset(x->variance_low, 0, sizeof(x->variance_low));

   if (xd->mb_to_right_edge < 0) pixels_wide += (xd->mb_to_right_edge >> 3);

@@ -999,7 +1003,7 @@

       }

       if (is_key_frame || (low_res &&

                            vt.split[i].split[j].part_variances.none.variance >

-                               (thresholds[1] << 1))) {

+                               threshold_4x4avg)) {

         force_split[split_index] = 0;

         // Go down to 4x4 down-sampling for variance.

         variance4x4downsample[i2 + j] = 1;