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 have always been confused by that--of the many functions in matplotlib.pyplot, `imshow` is the only one following that convention (there is no plotshow, histshow, pieshow). I'd be ok with just adding an alias..