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.

PanoPainter-OSX/Assets.xcassets/AppIcon.appiconset/Contents.json

@@ -43,13 +43,13 @@
     {
       "size" : "512x512",
       "idiom" : "mac",
-      "filename" : "icon512.png",
+      "filename" : "iTunesArtwork.png",
       "scale" : "1x"
     },
     {
       "size" : "512x512",
       "idiom" : "mac",
-      "filename" : "icon1024.png",
+      "filename" : "iTunesArtwork@2x.png",
       "scale" : "2x"
     }
   ],