[ot] Add TODO re zero-len MultipleSubst sequences

src/hb-ot-layout-gsub-table.hh

@@ -270,6 +270,15 @@ struct Sequence
   inline bool apply (hb_apply_context_t *c) const
   {
     TRACE_APPLY (this);
+
+    /* TODO:
+     * Testing shows that Uniscribe actually allows zero-len susbstitute,
+     * which essentially deletes a glyph.  We don't allow for now.  It
+     * can be confusing to the client since the cluster from the deleted
+     * glyph won't be merged with any output cluster...  Also, currently
+     * buffer->move_to() makes assumptions about this too.  Perhaps fix
+     * in the future after figuring out what to do with the clusters.
+     */
     if (unlikely (!substitute.len)) return TRACE_RETURN (false);
 
     unsigned int klass = _hb_glyph_info_is_ligature (&c->buffer->cur()) ?

src/hb-ot-layout-gsubgpos-private.hh

@@ -995,7 +995,9 @@ static inline bool apply_lookup (hb_apply_context_t *c,
 
     /* Recursed lookup changed buffer len.  Adjust. */
 
-    /* end can't go back past the current match position. */
+    /* end can't go back past the current match position.
+     * Note: this is only true because we do NOT allow MultipleSubst
+     * with zero sequence len. */
     end = MAX ((int) match_positions[idx] + 1, int (end) + delta);
 
     unsigned int next = idx + 1; /* next now is the position after the recursed lookup. */