threaded brush preview rendering

src/main.cpp

@@ -573,6 +573,8 @@ int main(int argc, char** argv)
         LOG("RegisterTouchWindow error: %s", GetLastErrorAsString().c_str());
     }
 
+    async_lock();
+
     LOG("init app");
     App::I.init();
 
@@ -604,6 +606,8 @@ int main(int argc, char** argv)
         LOG("SKIP init WinTab");
     }
 
+    async_unlock();
+
     LOG("change icon");
     SendMessage(hWnd, WM_SETICON, ICON_SMALL,
         (LPARAM)LoadIcon(GetModuleHandle(0), MAKEINTRESOURCE(IDI_ICON1)));