[set] Add hb_set_copy()

Fixes https://github.com/harfbuzz/harfbuzz/issues/3016

docs/harfbuzz-sections.txt

@@ -599,6 +599,7 @@ HB_SET_VALUE_INVALID
 hb_set_add
 hb_set_add_range
 hb_set_allocation_successful
+hb_set_copy
 hb_set_clear
 hb_set_create
 hb_set_del

src/hb-set.cc

@@ -172,6 +172,24 @@ hb_set_allocation_successful (const hb_set_t  *set)
   return set->successful;
 }
 
+/**
+ * hb_set_copy:
+ * @set: A set
+ *
+ * Allocate a copy of @set.
+ *
+ * Return value: Newly-allocated set.
+ *
+ * Since: REPLACEME
+ **/
+hb_set_t *
+hb_set_copy (const hb_set_t *set)
+{
+  hb_set_t *copy = hb_set_create ();
+  copy->set (set);
+  return copy;
+}
+
 /**
  * hb_set_clear:
  * @set: A set

src/hb-set.h

@@ -85,6 +85,9 @@ hb_set_get_user_data (hb_set_t           *set,
 HB_EXTERN hb_bool_t
 hb_set_allocation_successful (const hb_set_t *set);
 
+HB_EXTERN hb_set_t *
+hb_set_copy (const hb_set_t *set);
+
 HB_EXTERN void
 hb_set_clear (hb_set_t *set);