[gsubgpos] Keep another digest in the applicable_t

The digest for all the remaining subtables combined.
The idea is to get out of the subtable look as soon as
no more can be applied.

Doesn't seem to speed up anything I tested. Going to revert.

src/hb-ot-layout-gsubgpos.hh

@@ -1002,6 +1002,7 @@ struct hb_accelerate_subtables_context_t :
     hb_cache_func_t cache_func;
 #endif
     hb_set_digest_t digest;
+    hb_set_digest_t digest_rest;
   };
 
 #ifndef HB_NO_OT_LAYOUT_LOOKUP_CACHE
@@ -4339,6 +4340,12 @@ struct hb_ot_layout_lookup_accelerator_t
     thiz->digest.init ();
     for (auto& subtable : hb_iter (thiz->subtables, count))
       thiz->digest.add (subtable.digest);
+    if (count)
+      for (unsigned int i = count - 1; i; i--)
+      {
+	thiz->subtables[i - 1].digest_rest.add (thiz->subtables[i].digest);
+	thiz->subtables[i - 1].digest_rest.add (thiz->subtables[i].digest_rest);
+      }
 
 #ifndef HB_NO_OT_LAYOUT_LOOKUP_CACHE
     thiz->cache_user_idx = c_accelerate_subtables.cache_user_idx;
@@ -4358,23 +4365,28 @@ struct hb_ot_layout_lookup_accelerator_t
 #endif
   bool apply (hb_ot_apply_context_t *c, unsigned subtables_count, bool use_cache) const
   {
+    hb_codepoint_t g = c->buffer->cur().codepoint;
 #ifndef HB_NO_OT_LAYOUT_LOOKUP_CACHE
     if (use_cache)
     {
-      return
-      + hb_iter (hb_iter (subtables, subtables_count))
-      | hb_map ([&c] (const hb_accelerate_subtables_context_t::hb_applicable_t &_) { return _.apply_cached (c); })
-      | hb_any
-      ;
+      for (const auto &subtable : hb_iter (subtables, subtables_count))
+      {
+        if (subtable.apply_cached (c))
+	  return true;
+	if (!subtable.digest_rest.may_have (g))
+	  return false;
+      }
     }
     else
 #endif
     {
-      return
-      + hb_iter (hb_iter (subtables, subtables_count))
-      | hb_map ([&c] (const hb_accelerate_subtables_context_t::hb_applicable_t &_) { return _.apply (c); })
-      | hb_any
-      ;
+      for (const auto &subtable : hb_iter (subtables, subtables_count))
+      {
+        if (subtable.apply (c))
+	  return true;
+	if (!subtable.digest_rest.may_have (g))
+	  return false;
+      }
     }
     return false;
   }