also fix the z-index for the (deprecated) state-background overlay for it would otherwise obscure the slides in the overview mode from the mouse, i.e. clicking on a slide was not possible.
also fix the z-index for the (deprecated) state-background overlay for it would otherwise obscure the slides in the overview mode from the mouse, i.e. clicking on a slide was not possible.

GerHobbelt/reveal.js,GerHobbelt/reveal.js,GerHobbelt/reveal.js