[subset] Fix bug in composite operation counting.

Adding the result of add_gid_and_children to operation count over counts the number of operations.

src/hb-ot-glyf-table.hh

diff --git a/src/hb-ot-glyf-table.hh b/src/hb-ot-glyf-table.hh
index 517e603..6b419ea 100644
--- a/src/hb-ot-glyf-table.hh
+++ b/src/hb-ot-glyf-table.hh
@@ -1118,7 +1118,7 @@
       while (it)
       {
         auto item = *(it++);
-        operation_count +=
+        operation_count =
             add_gid_and_children (item.get_glyph_index (), gids_to_retain, depth, operation_count);
       }