[subset] add num-iterations flag to util/hb-subset.

util/hb-subset.cc

diff --git a/util/hb-subset.cc b/util/hb-subset.cc
index 97a3a2f..4f95578 100644
--- a/util/hb-subset.cc
+++ b/util/hb-subset.cc
@@ -98,7 +98,11 @@
   {
     hb_face_t *face = hb_font_get_face (font);
 
-    hb_face_t *new_face = hb_subset (face, input);
+    hb_face_t *new_face = nullptr;
+    for (unsigned i = 0; i < subset_options.num_iterations; i++) {
+      hb_face_destroy (new_face);
+      new_face = hb_subset (face, input);
+    }
     hb_blob_t *result = hb_face_reference_blob (new_face);
 
     failed = !hb_blob_get_length (result);

util/options-subset.cc

diff --git a/util/options-subset.cc b/util/options-subset.cc
index f1406c0..ed23bcd 100644
--- a/util/options-subset.cc
+++ b/util/options-subset.cc
@@ -252,6 +252,9 @@
     {"drop-tables", 0, 0, G_OPTION_ARG_CALLBACK,  (gpointer) &parse_drop_tables,  "Drop the specified tables.", "list of string table tags."},
     {"drop-tables+", 0, 0, G_OPTION_ARG_CALLBACK,  (gpointer) &parse_drop_tables,  "Drop the specified tables.", "list of string table tags."},
     {"drop-tables-", 0, 0, G_OPTION_ARG_CALLBACK,  (gpointer) &parse_drop_tables,  "Drop the specified tables.", "list of string table tags."},
+    {"num-iterations", 'n', 0, G_OPTION_ARG_INT,
+     &this->num_iterations,
+     "Run subsetter N times (default: 1)", "N"},
 
     {nullptr}
   };

util/options.hh

diff --git a/util/options.hh b/util/options.hh
index 30b8f5b..69465d0 100644
--- a/util/options.hh
+++ b/util/options.hh
@@ -679,6 +679,7 @@
   subset_options_t (option_parser_t *parser)
   {
     input = hb_subset_input_create_or_fail ();
+    num_iterations = 1;
     add_options (parser);
   }
 
@@ -689,6 +690,7 @@
 
   void add_options (option_parser_t *parser) override;
 
+  unsigned num_iterations;
   hb_subset_input_t *input;
 };