ocarina: Set volume button to current audio levels

When the button is created, it should be set to the current volume of
the gstreamer pipeline.  If I don't do this, the icon will default to a
"muted" status.

gui/buttons/volume.cpp

@@ -32,6 +32,8 @@ static void changed(GtkWidget *button, gdouble value, gpointer data)
 GtkWidget *make_volume_button()
 {
 	GtkWidget *volume  = gtk_volume_button_new();
+	gtk_scale_button_set_value(GTK_SCALE_BUTTON(volume),
+				   libsaria_get()->get_volume());
 	gtk_widget_show(volume);
 	set_increment(volume, 0.05);
 	resize(volume);