[gsubgpos] Optimize set-digest initialization

Previously we were once collecting set-digest for each subtable,
and another time for each lookup.

Now we compute the one for each lookup simply from the ones for
its subtables.

src/hb-ot-layout-gsubgpos.hh

@@ -3938,13 +3938,14 @@ struct hb_ot_layout_lookup_accelerator_t
   template <typename TLookup>
   void init (const TLookup &lookup)
   {
-    digest.init ();
-    lookup.collect_coverage (&digest);
-
     subtables.init ();
     OT::hb_accelerate_subtables_context_t c_accelerate_subtables (subtables);
     lookup.dispatch (&c_accelerate_subtables);
 
+    digest.init ();
+    for (auto& subtable : hb_iter (subtables))
+      digest.add (subtable.digest);
+
 #ifndef HB_NO_OT_LAYOUT_LOOKUP_CACHE
     cache_user_idx = c_accelerate_subtables.cache_user_idx;
     for (unsigned i = 0; i < subtables.length; i++)

src/hb-set-digest.hh

@@ -75,6 +75,8 @@ struct hb_set_digest_bits_pattern_t
 
   void init () { mask = 0; }
 
+  void add (const hb_set_digest_bits_pattern_t &o) { mask |= o.mask; }
+
   void add (hb_codepoint_t g) { mask |= mask_for (g); }
 
   bool add_range (hb_codepoint_t a, hb_codepoint_t b)
@@ -128,6 +130,12 @@ struct hb_set_digest_combiner_t
     tail.init ();
   }
 
+  void add (const hb_set_digest_combiner_t &o)
+  {
+    head.add (o.head);
+    tail.add (o.tail);
+  }
+
   void add (hb_codepoint_t g)
   {
     head.add (g);