[directwrite] Set unsafe to break flag

Like the rest of platform shapers. Otherwise hb-shape --verify goes
crazy.

src/hb-directwrite.cc

@@ -762,6 +762,8 @@ retry_getglyphs:
 
   if (isRightToLeft) hb_buffer_reverse (buffer);
 
+  buffer->unsafe_to_break_all ();
+
   delete [] clusterMap;
   delete [] glyphIndices;
   delete [] textProperties;