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_panel_brush.cpp

@@ -104,6 +104,7 @@ void NodePanelBrush::select_brush(int brush_id)
         {
             b->m_selected = true;
             m_current = b;
+            TextureManager::load(b->high_path.c_str(), true);
         }
     }
 }