Drawing into existing cairo context

Hello,

Is it possible for 'canvas' to support drawing into existing cairo context, for example `DrawingArea` in `node-gtk` library that supplies its own cairo context on the draw callback.

I poked a little bit and managed to make a hack to pass the context into the `CanvasRenderingContext2d` instance and get a simple clock example working. (see https://github.com/charobnjak32/node-canvas/commit/fe13e23057bec9323e930d5a2b8c4bdd45a77eaf)
However I have no knowledge in the cairo/gtk/nan libraries to understand whether there are some cases when this would break or whether this is a good idea or not.

Thanks.