settings: Provide a default style cascade per scale
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.
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