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.
Merge pull request #1 from mathias/support_subsprites

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