DrawingCtx.set_paint_source - new function, extracted from set_source_paint_server
DrawingCtx.set_paint_source - new function, extracted from set_source_paint_server

Let's see if we can avoid passing the ComputedValues of the node being
filtered to the filters engine.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg