MainWindow: fix window redraw after new project

Canvas is not initialized in application start so a window manager like x11 may reopen the mainwindow on gl context switching without this fix.

The behaviour was caused by different surface format / opengl contexts of the QMainWindow instance and the QOpenGLWidget instance of Canvas. In Qt6 it became more complex to quickly share the gl context. Hence we just create a dummy QOpenGLWidget before MainWindow is show to reset it.