Change the key-frame q mapping function

Increases the quality of key frames, but also reduces the
kf boost more when the key frame interval is small.

Results generally improve for SSIM, a little drop in
PSNR at short key frame intervals for low resolution videos.
However, visual quality shows a bigger improvement for several
YT encodes.

Results:
========
[lowres, cpu_used=1, 30 frames]:
Av PSNR 0.225%,	SSIM -0.570%

[lowres, cpu_used=1, 60 frames]:
Av PSNR 0.073%,	SSIM -0.575%

[midres, cpu_used=1, 30 frames]:
Av PSNR -0.023%, SSIM -0.314%

[midres, cpu_used=1, 60 frames]:
Av PSNR -0.348%, SSIM -0.582%

STATS_CHANGED

Change-Id: I729891693b4b0cfe9582e622de77bb546f981610

av1/encoder/ratectrl.c

diff --git a/av1/encoder/ratectrl.c b/av1/encoder/ratectrl.c
index 0fc2f6f..842d611 100644
--- a/av1/encoder/ratectrl.c
+++ b/av1/encoder/ratectrl.c
@@ -117,7 +117,7 @@
   for (i = 0; i < QINDEX_RANGE; i++) {
     const double maxq = av1_convert_qindex_to_q(i, bit_depth);
     kf_low_m[i] = get_minq_index(maxq, 0.000001, -0.0004, 0.150, bit_depth);
-    kf_high_m[i] = get_minq_index(maxq, 0.0000021, -0.00125, 0.55, bit_depth);
+    kf_high_m[i] = get_minq_index(maxq, 0.0000021, -0.00125, 0.45, bit_depth);
     arfgf_low[i] = get_minq_index(maxq, 0.0000015, -0.0009, 0.30, bit_depth);
     arfgf_high[i] = get_minq_index(maxq, 0.0000021, -0.00125, 0.55, bit_depth);
     inter[i] = get_minq_index(maxq, 0.00000271, -0.00113, 0.90, bit_depth);