missing mask for scroll event

This fixes https://github.com/matplotlib/matplotlib/issues/2668 
Apparently it is a know bug http://stackoverflow.com/questions/11775161/gtk3-get-mouse-scroll-direction https://bugzilla.gnome.org/show_bug.cgi?id=671305

Tried with example `examples/pylab_examples/image_slices_viewer.py` and works