EvPresentationView: Render correctly on hi-dpi displays

Create rendering jobs with a scale that incorporate the scale factor of
the widget, and then use cairo_surface_set_device_scale() to make the
resulting surfaces render at the correct size. Handle changes to the scale
factor both for the cached surfaces, and also for the monitor dimensions,
which are reported in scaled coordinates.

https://bugzilla.gnome.org/show_bug.cgi?id=723431