scrolling: interpret smooth deltas as pixels on quartz

This is a temporary workaround for scrolling units being amplified
on quartz, due to the assumption that smooth scrolling deltas are
always in some abstract unit similar to the one from xi2.

A proper solution for the situation is described in bug #736121, but
since we are close to release, this patch solves the issue temporarily.

https://bugzilla.gnome.org/show_bug.cgi?id=736121