Canvas manipulation from Python

[brentyi]

#58 adds the ability to export the canvas contents to a PNG.

It would be nice if we could, from Python:

• Read the render buffer / canvas contents (probably via request/response-type messages)
• Set the canvas width / height / aspect ratio
• Send files to download

This would be helpful for folks who want to (as an example) save a bunch of frames, stitch them into a video, and then send the video to the client to download.