Figure out how to adapt contact filters

[ericoporto]

My first guess is a contact stack that is read only and changes at each time step

I would also like to add some way to filter contacts with a fixture from this stack. And also, some way to pass an array of fixtures to ignore (which the contacts involving them aren't put on the array on first place).

Still, the callbacks approach is more interesting, but I am not sure how well it would work in AGS. Maybe need to branch and test this.