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vp9: Fix to active_best for GF/ARF in 1 pass vbr.

Correct the setting of Q basis of GF/ARF in 1 pass vbr.

Existing logic would switch to using avg_QP of key frame if
avg_QP of inter is less than active worst (even if key frame is
not last frame).

Instead fix the logic (as per the comment) to use the lower of
active_worst_quality and avg_Q for inter as basis for GF/ARF
active_best_quality (unless last frame was key frame).

Increase in metrics: AvgPSNR/SSIM up by ~0.7/0.3 on ytlive set.

Change-Id: I9a628378ec6684bfda9457ebfc2384ef6d8579f7

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

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

@@ -882,9 +882,12 @@

     // Use the lower of active_worst_quality and recent

     // average Q as basis for GF/ARF best Q limit unless last frame was

     // a key frame.

-    if (rc->frames_since_key > 1 &&

-        rc->avg_frame_qindex[INTER_FRAME] < active_worst_quality) {

-      q = rc->avg_frame_qindex[INTER_FRAME];

+    if (rc->frames_since_key > 1) {

+      if (rc->avg_frame_qindex[INTER_FRAME] < active_worst_quality) {

+        q = rc->avg_frame_qindex[INTER_FRAME];

+      } else {

+        q = active_worst_quality;

+      }

     } else {

       q = rc->avg_frame_qindex[KEY_FRAME];

     }