canvas: Use the window-scaling-factor setting
canvas: Use the window-scaling-factor setting

ClutterCanvas is a ClutterContent interface implementation; this means
that it can be created and modified regardless of whether it is
associated to a specific actor or a stage. For this reason, we cannot
walk the hierarchy and get the window scaling factor for high DPI
density displays out of the ClutterStage when we create the Cairo
surface that we will use to draw the canvas contents on.

We can use ClutterSettings:window-scaling-factor instead, since it's
what each ClutterStage will use anyway.

This will get slightly more complicated when we support per-output
window scaling factors (like on Wayland), but that will require changes
in the entire settings architecture anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=705915

GNOME/clutter,jigpu/clutter,jigpu/clutter,jigpu/clutter,GNOME/clutter,djdeath/clutter,Distrotech/clutter,GNOME/clutter,ebassi/clutter,ebassi/clutter,ebassi/clutter,ebassi/clutter,djdeath/clutter,djdeath/clutter,Distrotech/clutter,Distrotech/clutter,ebassi/clutter,djdeath/clutter,jigpu/clutter,djdeath/clutter,jigpu/clutter,GNOME/clutter,djdeath/clutter,GNOME/clutter,jigpu/clutter,Distrotech/clutter,Distrotech/clutter,Distrotech/clutter