Scene-cut detection: avoid very short KF intervals

In some videos with camera panning, scene cut algo incorrectly
identifies a scene cut after just a couple of frames.

We force a bigger keyframe interval to avoid this issue.

Large BDRate improvement on selected videos:
hdres/shields_720p50.y4m:       -10.717
hdres/stockholm_ter_720p60.y4m: -10.312
hdres/parkrun_720p50.y4m:       -3.501
hdres/mobcal_720p50.y4m:        -2.405
(other hdres vids neutral)

hdres overall: -0.709

lowres, midres, ugc360p and objective1-fast all neutral.

STATS_CHANGED

Change-Id: I601a306d2657f74ca8ebf5105f4cbf02121bf0ce

av1/encoder/pass2_strategy.c

diff --git a/av1/encoder/pass2_strategy.c b/av1/encoder/pass2_strategy.c
index 3e5f17f..b135938 100644
--- a/av1/encoder/pass2_strategy.c
+++ b/av1/encoder/pass2_strategy.c
@@ -1405,7 +1405,8 @@
   // Does the frame satisfy the primary criteria of a key frame?
   // See above for an explanation of the test criteria.
   // If so, then examine how well it predicts subsequent frames.
-  if ((this_frame->pcnt_second_ref < second_ref_usage_thresh) &&
+  if (frame_count_so_far >= 3 &&
+      (this_frame->pcnt_second_ref < second_ref_usage_thresh) &&
       (next_frame->pcnt_second_ref < second_ref_usage_thresh) &&
       ((this_frame->pcnt_inter < VERY_LOW_INTER_THRESH) ||
        ((pcnt_intra > MIN_INTRA_LEVEL) &&