[subset/cff2] Parse vsindexdict in the subset accelerator private dicts

The subset-side private-dict opset never handled OpCode_vsindexdict, so
accelerator_subset_t's privateDicts[fd].ivs always stayed 0 and charstring
interpretation during subsetting started from the wrong variation-store
index for any FD whose Private dict declares a non-zero vsindex.  Mirror
what cff2_private_dict_opset_t does; the op bytes are still recorded so
the verbatim-copy path is unchanged.

src/hb-ot-cff2-table.hh

diff --git a/src/hb-ot-cff2-table.hh b/src/hb-ot-cff2-table.hh
index 0618638..e46ea5a 100644
--- a/src/hb-ot-cff2-table.hh
+++ b/src/hb-ot-cff2-table.hh
@@ -346,6 +346,12 @@
 	env.clear_args ();
 	break;
 
+      case OpCode_vsindexdict:
+	env.process_vsindex ();
+	dictval.ivs = env.get_ivs ();
+	env.clear_args ();
+	break;
+
       default:
 	SUPER::process_op (op, env);
 	if (!env.argStack.is_empty ()) return;