[util/hb-subset] Add --layout to keep GDEF/GSUB/GPOS

Will become default and option removed in the future.
diff --git a/util/hb-subset.cc b/util/hb-subset.cc
index 9eb95bb..b7d9eb9 100644
--- a/util/hb-subset.cc
+++ b/util/hb-subset.cc
@@ -89,6 +89,7 @@
 
   void finish (const font_options_t *font_opts)
   {
+    hb_subset_input_set_drop_layout (input, !subset_options.keep_layout);
     hb_subset_input_set_drop_hints (input, subset_options.drop_hints);
     hb_subset_input_set_desubroutinize (input, subset_options.desubroutinize);