Improved Force Key Frame Behaviour

These changes improve the behaviour of the code with
forced key frames sent in by a calling application.

The sizing of the frames is still suboptimal for two pass in
particular but the behaviour is much better than it was.

Change-Id: I35fae610c67688ccc69d11f385e87dfc884e65a1

vp8/encoder/ratectrl.c

diff --git a/vp8/encoder/ratectrl.c b/vp8/encoder/ratectrl.c
index d328081..3712722 100644
--- a/vp8/encoder/ratectrl.c
+++ b/vp8/encoder/ratectrl.c
@@ -1444,6 +1444,9 @@
     }
     else
     {
+        int last_kf_interval =
+                (cpi->frames_since_key > 0) ? cpi->frames_since_key : 1;
+
         // reset keyframe context and calculate weighted average of last KEY_FRAME_CONTEXT keyframes
         for (i = 0; i < KEY_FRAME_CONTEXT; i++)
         {
@@ -1454,8 +1457,8 @@
             }
             else
             {
-                cpi->prior_key_frame_size[KEY_FRAME_CONTEXT - 1]     = cpi->projected_frame_size;
-                cpi->prior_key_frame_distance[KEY_FRAME_CONTEXT - 1] = cpi->frames_since_key;
+                cpi->prior_key_frame_size[i]     = cpi->projected_frame_size;
+                cpi->prior_key_frame_distance[i] = last_kf_interval;
             }
 
             av_key_frame_bits      += prior_key_frame_weight[i] * cpi->prior_key_frame_size[i];