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/app_layout.cpp

@@ -3,6 +3,7 @@
 #include "node_icon.h"
 #include "node_dialog_open.h"
 #include "node_text.h"
+#include "node_progress_bar.h"
 
 using namespace ui;
 
@@ -528,7 +529,9 @@ void App::initLayout()
         canvas->m_brush = b;
 
         brush_update();
-
+        
+        TextureManager::load("data/paper.jpg");
+        
         // hacky thing to make the toolbar buttons not steal events when moving cursor fast
         if (auto* toolbar = layout[main_id]->find<Node>("toolbar"))
             toolbar->m_flood_events = true;