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vp9: Reduce frame loopfilter-level for 1 pass cbr.

Reduce the filt_guess for 1 pass cbr on inter-frames.
This reduces visual artifact seen in rtc clip (jimred.vga),
and improves metrics on rtc set.

Metrics on rtc set for cbr mode overall positive, most clips are up:
Speed 7 rtc: avgPSNR/SSIM up by: ~2.6/3.9%
Speed 8 rtc: avgPSNR/SSIM up by: ~1.3/2.5%

Change-Id: Ia4eccea1c19d65b583516df28823cd756c49464f

--- a/vp9/encoder/vp9_picklpf.c

+++ b/vp9/encoder/vp9_picklpf.c

@@ -180,6 +180,10 @@

     }

 #else

     int filt_guess = ROUND_POWER_OF_TWO(q * 20723 + 1015158, 18);

+    if (cpi->oxcf.pass == 0 && cpi->oxcf.rc_mode == VPX_CBR &&

+        cpi->oxcf.content != VP9E_CONTENT_SCREEN && cm->frame_type != KEY_FRAME)

+      filt_guess = 5 * filt_guess >> 3;

+

 #endif  // CONFIG_VP9_HIGHBITDEPTH

     if (cm->frame_type == KEY_FRAME) filt_guess -= 4;

     lf->filter_level = clamp(filt_guess, min_filter_level, max_filter_level);