[Normative, jnt_comp] Add sequence header to turn on/off

Add one bit in sequence header to turn on/off jnt_comp.
Also add encoder control.

BUG=aomedia:1435

Change-Id: I8c72b78bed6c8ae9da4d4eeba4635e0a619acf80
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index b4058fa..7b84174 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -3091,6 +3091,10 @@
 
   aom_wb_write_bit(wb, seq_params->enable_dual_filter);
 
+#if CONFIG_JNT_COMP
+  aom_wb_write_bit(wb, seq_params->enable_jnt_comp);
+#endif
+
   if (seq_params->force_screen_content_tools == 2) {
     aom_wb_write_bit(wb, 1);
   } else {