gtk3: use Cairo::Context#source instead of #target

Because the following error occurred:

    /.../gtk3/sample/gtk-demo/drawingarea.rb:141:in `target': the target surface has been finished
             from /.../gtk3/sample/gtk-demo/drawingarea.rb:141:in `scribble_draw'
             from /.../gtk3/sample/gtk-demo/drawingarea.rb:72:in `block in initialize'
             from ./main.rb:384:in `call'
             from ./main.rb:384:in `main'
             from ./main.rb:384:in `<main>'