Add an experiment to disable lpf on tile boundaries

This commit adds a new experiment to allow disabling of loop filtering
on tile boundaries. It is implemented by adding a syntax field
"loopfilter_across_tiles_enabled" into the uncompressed frame header. 
If it is set to 0, decoder and encoder will disables loop filtering for
block edges that are also tile boundaries.

Change-Id: Ib80bfd82d49c74f1ba46ae18ceedb30704ac8aa5
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index d721e53..0cb3bf1 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -3382,6 +3382,10 @@
   aom_wb_write_bit(wb, cm->log2_tile_rows != 0);
   if (cm->log2_tile_rows != 0) aom_wb_write_bit(wb, cm->log2_tile_rows != 1);
 #endif  // CONFIG_EXT_TILE
+
+#if CONFIG_DEBLOCKING_ACROSS_TILES
+  aom_wb_write_bit(wb, cm->loop_filter_across_tiles_enabled);
+#endif  // CONFIG_DEBLOCKING_ACROSS_TILES
 }
 
 static int get_refresh_mask(AV1_COMP *cpi) {