Add a fillStyle option to EnabledElementOptions.

interface EnabledElementOptions {
  renderer?: 'webgl';
  fillStyle?: 'black' | 'white';
}

If options.fillStyle is not set, the same with default behavior.

context.fillStyle = 'black';

If options.fillStyle is set to white or black, you can set default fillStyle for different element, and the fillStyle will responsible to viewport.invert.

const enabledElementOptions = { renderer: 'webgl' };
enabledElementOptions.fillStyle = modality === 'PT' ? 'white' : 'black';
cornerstone.enable(element, enabledElementOptions);

I still have a bug here, I can't figure out it, when I have not set renderer to webgl and set the fillStyle to white, the viewport will render to all white.🤔️ I need your help @dannyrb for finishing this PR.