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vp9: Adjustment to active_best_quality for inter_frame, 1 pass vbr.

Change only affects 1 pass vbr.

Use a q value somewhat larger (~6%) than avg_frame_qindex[INTER]
as basis for active_best_quality for inter-frames.
And use the minium of this (avg_frame_qindex) and the active_worst_quality.

This reduces some overshoot in ytlive clips.
Overall small but positive average increase in metrics (up on average ~0.2%).

Change-Id: Icdbaae7872d5675fd38a13c0ec6ce0e2e3b919ce

--- a/vp9/encoder/vp9_ratectrl.c

+++ b/vp9/encoder/vp9_ratectrl.c

@@ -948,11 +948,15 @@

                              FIXED_GF_INTERVAL], cm->bit_depth);

       active_best_quality = VPXMAX(qindex + delta_qindex, rc->best_quality);

     } else {

-      // Use the lower of active_worst_quality and recent/average Q.

-      if (cm->current_video_frame > 1)

-        active_best_quality = inter_minq[rc->avg_frame_qindex[INTER_FRAME]];

-      else

+      // Use the min of the average Q (with some increase) and

+      // active_worst_quality as basis for active_best.

+      if (cm->current_video_frame > 1) {

+        q = VPXMIN(((17 * rc->avg_frame_qindex[INTER_FRAME]) >> 4),

+                    active_worst_quality);

+        active_best_quality = inter_minq[q];

+      } else {

         active_best_quality = inter_minq[rc->avg_frame_qindex[KEY_FRAME]];

+      }

       // For the constrained quality mode we don't want

       // q to fall below the cq level.

       if ((oxcf->rc_mode == VPX_CQ) &&