This guarantees we only create 1 extra style cascade in total for hidpi
and not one per style context.
Style cascades are now nested like this:
GtkSettings root cascade (scale == 1)
|
+-- GtkSettings per scale cascade (for any scale, no custom providers)
|
+-- GtkStyleContext custom cascade (for any scale, custom providers)
This requires a bunch of care when changing cascade-related properties
inside GtkStyleContext, so that it ends up with a properly setup
cascade, but I think I got those cases right.
The only thing we don't do yet is reverting to a GtkSettings cascade
when the last custom provider is removed from a custom cascade.
Settings have a little more metadata than plain properties. They
can come from different sources. Make this information available
so we can show it in the inspector.
https://bugzilla.gnome.org/show_bug.cgi?id=736971
