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.
> I imagined the ultimate plan for vispy.mpl_plot would be to re-implement some of the most commonly used functions without matplotlib..

Yes, this is also a possibility: put pyplot stuff in `mpl_plot` exclusively.