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 seems weird to have a `clear` argument for `draw_visual`. Is this function usually meant to be used to draw things in isolation? Needs docstring for these arguments anyway, so this could be clarified there.