Support SVG2 context-fill and context-stroke

SVG2's paint specifications (for example, as used for the fill and stroke properties) can now have the values context-fill and context-stroke (Specifying Paint).

When used as the paint for a context element — something inside <marker> or <use>, these means to use the fill or stroke of the shape referencing the marker, or the <use> element itself.

That last link mentions, "If the referenced value of fill or stroke is a context-fill and context-stroke value, then those contextual referencing is recursive". I think DrawingCtx could carry around a stack of fill and stroke values whose top element would be used when the context-fill and context-stroke values are used. Note that these are only supposed to work when inside a <marker> or <use>, that last link also mentions "If there is no context element and these keywords are used, then no paint is applied.".