plug-ins: WebP kmax hint text not properly displayed for origin value.

The hint text was shown on kmax change, but not when it was 0 or 1 from
start (i.e. when previous export was 0/1 so these were the saved
values). Run the callback upon construction to initialize properly this
hint label.

plug-ins/file-webp/file-webp-dialog.c

@@ -302,6 +302,7 @@ save_dialog (WebPSaveParams *params,
       g_signal_connect (adj_kf, "value-changed",
                         G_CALLBACK (show_maxkeyframe_hints),
                         label_kf);
+      show_maxkeyframe_hints (adj_kf, GTK_LABEL (label_kf));
 
       /* minimize-size checkbox */
       toggle_minsize = gtk_check_button_new_with_label (_("Minimize output size (slower)"));