Add blend function support.

Allow use shader in a single layer.

Anm2player will now read the width and height information in the key frame of the null layer.

Updated dom4j to 2.1.3 for a security problem.

Solves the problem of asynchronous animation between layers.

Optimized the algorithm of tween animation.

You can now customize the keyframe interpolation method for each layer.

Add support for null animation layer.
Add getter for some fields.
Change some fields' visibility to public.