Added beginRender lifecycle event.

[joegaffey]

I needed to render a background image directly onto the canvas so that I could capture gameplay video including backgrounds using HTMLCanvasElement.captureStream() in my physics based game: https://joegaffey.itch.io/bugvsboxes

I would like to add background animations later that would also benefit from this event.
As a fairly simple game I chose not to include another canvas renderer - matter.js is the only dependency.

Tests indicate no performance impact.