+3 new filters, resize & combine textures

* Three new filter types were added, mainly for texture editing
purposes: ColorOverlayFilter, ResizeFilter, ScaleFilter.
- Textures can be resized and scaled easily, either by passing an
instance of the filters mentioned above to Texture.filter() or by
calling the shorthand methods Texture.scale() / Texture.resize().
* Two instances of Texture can also be combined now, by calling
Texture.combine() with the one to draw on top as the parameter.