Refactor gop structure data access

Introduce FRAME_INFO data structure to host the original source
frame information. Localize the data access scope in GOP structure
decision process.

Change-Id: I87d72f7fa9913dd584208a2a93d138d375b0871c
diff --git a/av1/encoder/encoder.h b/av1/encoder/encoder.h
index f1ba105..e59b9bc 100644
--- a/av1/encoder/encoder.h
+++ b/av1/encoder/encoder.h
@@ -924,6 +924,8 @@
 
   int droppable;
 
+  FRAME_INFO frame_info;
+
   int initial_width;
   int initial_height;
   int initial_mbs;  // Number of MBs in the full-size frame; to be used to