Do offline render in a thread, show render progress etc

Source/PluginProcessor.h

@@ -162,6 +162,7 @@ public:
 	ValueTree getStateTree(bool ignoreoptions, bool ignorefile);
 	void setStateFromTree(ValueTree tree);
 	String offlineRender(File outputfile);
+	std::atomic<int> m_offline_render_state{ -1 };
 	bool m_state_dirty = false;
 	std::unique_ptr<AudioThumbnail> m_thumb;
 	bool m_show_technical_info = false;