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.
It's standard across `numpy`, `scipy`, `sklearn`, etc. (so expected by users / semi-standard), we have it formatted this way everywhere else, is probably necessary for our docs to build properly, etc.