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/canvas.h

@@ -41,12 +41,13 @@ public:
 
 class Canvas
 {
+public:
     Plane m_plane;
     Plane m_plane_brush;
     BrushMesh m_mesh;
     bool m_dirty = false;
     bool m_commit_delayed = false;
-public:
+
     static Canvas* I;
     bool m_alpha_lock = false;
     bool m_touch_lock = true;
@@ -67,6 +68,7 @@ public:
     Layer m_smask; // selection mask
     bool m_smask_active = false;
     RTT m_tmp[6];
+    Texture2D m_brush_mix;
     Texture2D m_tex[6];
     Texture2D m_tex2[6];
     bool m_pick_ready[6];
@@ -79,6 +81,7 @@ public:
     Sampler m_sampler_brush;
     Sampler m_sampler_bg;
     Sampler m_sampler_mask;
+    Sampler m_sampler_stencil;
     glm::vec2 m_cam_rot;
     glm::vec3 m_cam_pos;
     float m_cam_fov = 85;
@@ -125,9 +128,12 @@ public:
     void snap_history(const std::vector<int>& planes);
     void clear_context();
     void export_equirectangular(std::string data_path);
+    void export_equirectangular_thread(std::string data_path);
     void export_anim(std::string data_path);
     void project_save(std::string data_path);
+    void project_save_thread(std::string data_path);
     void project_open(std::string data_path);
+    void project_open_thread(std::string data_path);
     void inject_xmp(std::string jpg_path);
     ui::Image thumbnail_generate(int w, int h);
     ui::Image thumbnail_read(std::string data_path);