A way to disable premultipliedAlpha on transparent webGL context

This is a cross-renderer way to disable premultipliedAlpha on webGL renderer. You just pass `notMultiplied` as `transparent` parameter. This way it works with renderer detection and is ignored gracefully.

```js
new PIXI.WebGLRenderer(width, height, canvas, 'notMultiplied');
```

Rationale: Disabling premultipliedAlpha is required if you want to export the transparent canvas with toDataURI/toBlob