[OT] Port Arabic fallback shaping to synthetic GSUB

All of init/medi/fina/isol and rlig implemented.

Let there be dragons... ⻯
diff --git a/src/hb-ot-map.cc b/src/hb-ot-map.cc
index aae4d03..f290c98 100644
--- a/src/hb-ot-map.cc
+++ b/src/hb-ot-map.cc
@@ -236,6 +236,7 @@
 	m.global_mask |= (info->default_value << map->shift) & map->mask;
     }
     map->_1_mask = (1 << map->shift) & map->mask;
+    map->needs_fallback = !found;
 
   }
   feature_infos.shrink (0); /* Done with these */