[cairo] pixman box filtering code prototype

[Frédéric Plourde]

> I've attached a patch that has a rough implementation of this.
>   
your code in  /pixman-rescale-box.c  looks good to me, but why haven't 
you placed your filtering calls down to pixman-transformed.c ?  it seems 
to me that your box filtering should belong to where former normal 
bilinear filtering code was implemented.... take a look at 
pixman-transformed.c

-fred-