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.
And note the call to `gloo.clear` in `on_draw`, which can probably be removed. Actually, why do we need `render()` to be public? Can the code that needs it not simply call `on_draw()`?