Search for deblocking filter level for superblock

Search and pick filter level for each superblock after the entire
frame has been encoded. And then apply loop filtering using
selected filter levels.

Now it can't work with LOOPFILTER_LEVEL. Y, U, V planes share the
same filter level.

Per superblock, filter levels are signaled via bitstream.

Future work: move selecting filter level into each superblock
rather than wait for the whole frame encoded.

Change-Id: I46cb994ec4866abb0456146f7c3cae8813c1d52e
diff --git a/av1/common/blockd.h b/av1/common/blockd.h
index 5262d24..c381e3a 100644
--- a/av1/common/blockd.h
+++ b/av1/common/blockd.h
@@ -494,6 +494,9 @@
 #endif
 
   BOUNDARY_TYPE boundary_info;
+#if CONFIG_LPF_SB
+  uint8_t filt_lvl;
+#endif
 } MB_MODE_INFO;
 
 typedef struct MODE_INFO {