Store pre_fc in AV1_COMMON rather than computing it on the spot.

Prep work for changing how frame contexts are signaled.

Change-Id: I0db8ec631fc2d616d16fabaf72eac5663ffe3630
diff --git a/av1/common/onyxc_int.h b/av1/common/onyxc_int.h
index b60112e..aa6f212 100644
--- a/av1/common/onyxc_int.h
+++ b/av1/common/onyxc_int.h
@@ -331,6 +331,7 @@
 
   FRAME_CONTEXT *fc;              /* this frame entropy */
   FRAME_CONTEXT *frame_contexts;  // FRAME_CONTEXTS
+  FRAME_CONTEXT *pre_fc;          // Context referenced in this frame
   unsigned int frame_context_idx; /* Context to use/update */
   FRAME_COUNTS counts;