We need to provide an escape hatch to ClutterCanvas so that it's
possible to override the window-scaling-factor ClutterSetting. This is
going to be useful in the future in case the user has better knowledge
of the window scaling factor that is going to be used with a specific
set of ClutterCanvas contents (e.g. on different outputs or stages).
https://bugzilla.gnome.org/show_bug.cgi?id=705915
