More conservative rule to enable intrabc

Adjust the threshold that is used to adaptively turn on/off intrabc.
This reduces the possibility that regular videos are falsely identified
as screen content.

Change-Id: If14c90c8a209ba9d5e3a4e63f2932ce10fe1767a

av1/encoder/encoder.c

diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index 53077bb..3dc49f4 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -3510,7 +3510,7 @@
   // IntraBC would force loop filters off, so we use more strict rules that also
   // requires that the block has high variance.
   cm->allow_intrabc = cm->allow_screen_content_tools &&
-                      counts_2 * blk_h * blk_w * 15 > width * height;
+                      counts_2 * blk_h * blk_w * 12 > width * height;
 }
 
 static void set_size_independent_vars(AV1_COMP *cpi) {