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_canvas.h

@@ -9,6 +9,7 @@ public:
     std::unique_ptr<ui::Canvas> m_canvas;
     ui::Brush m_brush;
     Sampler m_sampler;
+    Sampler m_sampler_stencil;
     ui::Plane m_face_plane;
     ui::LineSegment m_line;
     virtual Node* clone_instantiate() const override;