client/gtk2/ibusimcontext: Fix forward key keycode for GTK4

When a keycode is provided (!= 0) for a forwarded key event, convert it to a
GTK keycode before passing it to gtk_im_context_filter_key().

Also free GdkKeymapKey after gdk_display_map_keyval() is called.

BUG=#2380
BUG=#2382

client/gtk2/ibusimcontext.c

@@ -1945,14 +1945,17 @@ _ibus_context_forward_key_event_cb (IBusInputContext  *ibuscontext,
 #if GTK_CHECK_VERSION (3, 98, 4)
     int group = 0;
     g_return_if_fail (GTK_IS_IM_CONTEXT (ibusimcontext));
-    if (keycode == 0 && ibusimcontext->client_window) {
+    if (keycode != 0) {
+        keycode += 8; // to GTK keycode
+    } else if (ibusimcontext->client_window) {
         GdkDisplay *display =
                 gtk_widget_get_display (ibusimcontext->client_window);
         GdkKeymapKey *keys = NULL;
         gint n_keys = 0;
         if (gdk_display_map_keyval (display, keyval, &keys, &n_keys)) {
             keycode = keys->keycode;
             group = keys->group;
+            g_free (keys);
         } else {
             g_warning ("Failed to parse keycode from keyval %x", keyval);
         }