implement multithreaded rendering with context switch, gl state save/restore, add progress bar ui node, implement stencil texture for brush, implement multithreaded canvas load/save/export pano. Missing multithread in windows.

engine/node_progress_bar.h

@@ -0,0 +1,16 @@
+#pragma once
+#include "node.h"
+#include "node_button.h"
+#include "node_text.h"
+#include "node_border.h"
+
+class NodeProgressBar : public NodeBorder
+{
+public:
+    Node* m_template;
+    NodeButton* btn_cancel;
+    NodeText* m_title;
+    NodeBorder* m_progress;
+    virtual Node* clone_instantiate() const override;
+    virtual void init() override;
+};