[subset] Add layout_scripts to subset input.

src/hb-subset-input.cc

@@ -203,6 +203,8 @@ hb_subset_input_create_or_fail (void)
 
   input->sets.layout_features->add_array (default_layout_features, ARRAY_LENGTH (default_layout_features));
 
+  input->sets.layout_scripts->invert (); // Default to all scripts.
+
   if (input->in_error ())
   {
     hb_subset_input_destroy (input);

src/hb-subset-input.hh

@@ -50,6 +50,7 @@ struct hb_subset_input_t
     hb_set_t *name_ids;
     hb_set_t *name_languages;
     hb_set_t *layout_features;
+    hb_set_t *layout_scripts;
   };
 
   union {

src/hb-subset-plan.cc

@@ -555,8 +555,7 @@ hb_subset_plan_create_or_fail (hb_face_t	 *face,
   _nameid_closure (face, plan->name_ids);
   plan->name_languages = hb_set_copy (input->sets.name_languages);
   plan->layout_features = hb_set_copy (input->sets.layout_features);
-  plan->layout_scripts = hb_set_create ();
-  hb_set_invert (plan->layout_scripts);
+  plan->layout_scripts = hb_set_copy (input->sets.layout_scripts);
   plan->glyphs_requested = hb_set_copy (input->sets.glyphs);
   plan->drop_tables = hb_set_copy (input->sets.drop_tables);
   plan->no_subset_tables = hb_set_copy (input->sets.no_subset_tables);

src/hb-subset.h

@@ -100,6 +100,8 @@ typedef enum { /*< flags >*/
  * @HB_SUBSET_SETS_NAME_LANG_ID: the set of name lang ids that will be retained.
  * @HB_SUBSET_SETS_LAYOUT_FEATURE_TAG: the set of layout feature tags that will be retained
  * in the subset.
+ * @HB_SUBSET_SETS_LAYOUT_SCRIPT_TAG: the set of layout script tags that will be retained
+ * in the subset. Defaults to all tags.
  *
  * List of sets that can be configured on the subset input.
  *
@@ -113,6 +115,7 @@ typedef enum {
   HB_SUBSET_SETS_NAME_ID,
   HB_SUBSET_SETS_NAME_LANG_ID,
   HB_SUBSET_SETS_LAYOUT_FEATURE_TAG,
+  HB_SUBSET_SETS_LAYOUT_SCRIPT_TAG,
 } hb_subset_sets_t;
 
 HB_EXTERN hb_subset_input_t *