[OTLayout] Don't recurse in collect_glyphs() for GPOS

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

@@ -1446,7 +1446,7 @@ struct PosLookup : Lookup
   inline hb_collect_glyphs_context_t::return_t collect_glyphs_lookup (hb_collect_glyphs_context_t *c) const
   {
     TRACE_COLLECT_GLYPHS (this);
-    c->set_recurse_func (process_recurse_func<hb_collect_glyphs_context_t>);
+    c->set_recurse_func (NULL);
     return TRACE_RETURN (process (c));
   }
 

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

@@ -156,6 +156,10 @@ struct hb_collect_glyphs_context_t
     if (unlikely (nesting_level_left == 0 || !recurse_func))
       return default_return_value ();
 
+    /* Note that GPOS sets recurse_func to NULL already, so it doesn't get
+     * past the previous check.  For GSUB, we only want to collect the output
+     * glyphs in the recursion.  If those are not requested, we can go home now. */
+
     nesting_level_left--;
     /* Only collect output glyphs in the recursion. */
     hb_collect_glyphs_context_t new_c (this->face, NULL, NULL, NULL, &output, nesting_level_left);