Don't use 'window' if it isn't available

[haroldiedema]

You can no longer instantiate the WebGLRenderer in a worker context since r93 because of the WebGLAnimation things that have been added to the WebGLRenderer constructor code.

It blatantly assumes there is a window object available, which results in an immediate reference error as soon as you instantiate the renderer.

This PR fixes that, although it will result in errors when trying to use the onAnimationFrame functionality since it also assumes requestAnimationFrame is available in a worker context.

Then again, using requestAnimationFrame from a worker context doesn't make much sense anyway. 😄

[mrdoob]

How about doing this instead?

if ( typeof window !== 'undefined' ) animation.setContext( window );

[mrdoob]

Thanks!