Improve performance

[natano]

• Switch from socket.io to plain websockets to reduce CPU load in the browser as well as on server side
• Don't check canvas dimensions on every frame, but use the window resize event instead
• Move (potentially) expensive computations out of the render loop
• Cap frame rate at 60 FPS. That should prevent excessive CPU usage with high monitor refresh rates (e.g. my 144 Hz monitor at home)

[teejusb]

Can we also update the README.md to indicate we need Python 3.7+ instead of 3.6+ (because of asyncio)