If the scale has an origin (it will have one by default), GtkRange will
render the two sides before/after the current value with different style
classes, making it possible for themes to use different colors and
properties for the two areas.
This was possible in GTK 2 with style details, but got lost during the
road to 3.0.
https://bugzilla.gnome.org/show_bug.cgi?id=665140