GLVolume: split composite_id to object_id, volume_id and instance_id.

WIP: Added creation of GLVolumes for SLA supports.

src/slic3r/GUI/BackgroundSlicingProcess.cpp

@@ -235,6 +235,12 @@ void BackgroundSlicingProcess::stop_internal()
 	m_print->set_cancel_callback([](){});
 }
 
+bool BackgroundSlicingProcess::empty() const
+{
+	assert(m_print != nullptr);
+	return m_print->empty();
+}
+
 std::string BackgroundSlicingProcess::validate()
 {
 	assert(m_print != nullptr);