Support the use of sprite sheets through sub-rectangle drawImage syntax

- See https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D?redirectlocale=en-US&redirectslug=DOM%2FCanvasRenderingContext2D#drawImage() for parameters and documentation.

It was necessary to add this for my game, so I thought I would contribute it back. Certainly open to make the parameter names more readable if that is a concern.