Wait for work to be finished before shutting down

Stopping the timers isn't sufficient, also wait for in-flight work to be
finished first.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

source/tools/atlas/AtlasUI/ScenarioEditor/ScenarioEditor.cpp

@@ -693,6 +693,9 @@ void ScenarioEditor::OnClose(wxCloseEvent& event)
 
 	m_Timer.Stop();
 	m_RenderTimer.Stop();
+	// Wait for in-flight work to finish.
+	qPing qry;
+	qry.Post();
 
 	m_ToolManager.SetCurrentTool(_T(""));