Increase Minimum KF boost.

This increase in the minimum KF boost impacts only very complex clips
and in practice mainly impacts smaller image formats.

Using 150 frames VBR over typical YT data rates their was only a very small
effect for the midres and hd res sets.

For the Lowres data set the results were as follows (% -ve = better)
-0.037, -0.031, -0.112, -0.043 (av psnr, overal psnr, ssim, psnr-hvs)

Even on this set most clips see no change. The largest change is Husky
(-1.093, -0.772, -2.305, -1.363)


Change-Id: I6d0e9fd986e0ecd563384f080a7740d627a55b56
diff --git a/av1/encoder/pass2_strategy.c b/av1/encoder/pass2_strategy.c
index 735acc8..ad1c928 100644
--- a/av1/encoder/pass2_strategy.c
+++ b/av1/encoder/pass2_strategy.c
@@ -1346,7 +1346,7 @@
 #define KF_MIN_FRAME_BOOST 80.0
 #define KF_MAX_FRAME_BOOST 128.0
-#define MIN_KF_BOOST 300          // Minimum boost for non-static KF interval
+#define MIN_KF_BOOST 600          // Minimum boost for non-static KF interval
 #define MIN_STATIC_KF_BOOST 5400  // Minimum boost for static KF interval
 static void find_next_key_frame(AV1_COMP *cpi, FIRSTPASS_STATS *this_frame) {