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Previously a style cascade's parent could not have a parent itself. That represented the two levels at which you could add a style provider: at the screen level, with gtk_style_context_add_provider_for_screen(), and at the style context level, with gtk_style_context_add_provider(). This commit changes no functionality, but this change will be necessary for adding style providers in the future that apply to a subtree of the widget tree. It relaxes the requirement that a style cascade's parent must not have a parent, since in the future a style context may be affected by any number of parent widgets' style contexts. https://bugzilla.gnome.org/show_bug.cgi?id=751409
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