gsk: Make GskRenderer.render() drawing context-agnostic
We're going to need to allow rendering on a specific cairo_t in order to implement fallback code paths inside GTK; this means that there will be times when we have a transient GskRenderer instance that does not have a GdkDrawingContext to draw on. Instead of adding a new render() implementation for those cases and then decide which one to use, we can remove the drawing context argument from the virtual function itself, and allow using a NULL GdkDrawingContext when calling gsk_renderer_render(). A later commit will add a generic function to create a transient GskRenderer with a cairo_t attached to it. Renderers inside GSK will have to check whether we have access to a GdkDrawingContext, in which case we're going to use it; or if we have access to a cairo_t and a window.
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