Remove explicit use of `window`

[RealOrangeOne]

Window as a global doesnt exist when run under Node.JS. However, if run under something like jsdom, it does.

This PR allows window to be accessed if it exists, but stops anything breaking if it doesnt exist.

Should fix #69

[lelandrichardson]

@RealOrangeOne what about just using global, since any bundler that runs this code in the browser will use window for that.

if (global && global.cancelAnimationFrame) { ... }

[lelandrichardson]

LGTM