Add SceneCanvas.screenshot method

Just a start. It implements the ability to render any portion of the canvas at any resolution. 
This is also a good test to ensure that visuals are correctly handling the doc / framebuffer / ndc coordinate systems.

To do:
* [ ] Automatic tiled rendering for very high resolution
* [x] Disentangle logic between `render` and `screenshot` to avoid warning messages.
Maybe we need three interfaces then?

* mpl_plot: fully compatible with mpl, because it *is* mpl with a hook
* py_plot: almost compatible with mpl, but does not depend on it
* plot: a modern API