Added MaskedFillSprite component

The MaskedFillSprite allows the creation of arbitrary colored shapes by masking a fillsprite with a given texture.

This is useful for creating shapes with dynamic colours, e.g. a mauve hat:

```haxe
var mauve = 0xe0b0ff;
var hat   = pack.getTexture('hat.png');
new MaskedFillSprite(mauve, hat);
```