Added Convolution Filter

This adds a convolution filter which accepts a matrix (specified as a 9 point Array) It can be used to create many further effects such as sharpen, emboss, blurs, edge detection and more.  see http://docs.gimp.org/en/plug-in-convmatrix.html for more info.

As you can see it currently requires three parameters:

```
matrix = [
        0, -1, 0,
        -1, 5, -1,
        0, -1, 0
    ];

width = texture width;
height = texture height;
```

Ideally we should be able to access the texture dimensions inside the filter class but I couldn't find anywhere to do so for now I have left them as parameters to pass.  I am very inexperienced with pixi.js and didn't have time to go poking about too much.

I have already created sharpen, emboss, edge detection and edge enhance filters with this.