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.
Ah, I see. Gloo gives a warning because it wants to keep a local copy and wants it to be contiguous... Another way to avoid this is to initialize the texture using `store=False`. Or we can wait for the gloo refactoring...