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.
On the other hand, having a pyplot without relying on mpl would be nice. I suppose we need to discuss the scope and ideas for `vispy.plot` someday soon. Maybe also ask for user input.