Option to control premultiplied alpha on textures

I've added an option to `BaseTexture` controlling `gl.UNPACK_PREMULTIPLY_ALPHA_WEBGL` for the texture. 
This allows to create shaders that rely on untouched RGB values. It's meant to be used as follows:

```
texture = PIXI.Texture.fromImage(url);
texture.baseTexture.premultipliedAlpha = false;
```

It defaults to `true`, so it doesn't break anything.