Handle kicker holes in playfield

Currently the playfield's material is cut-out, meaning that if there's no texture, it's opaque. This results in the kickers being virtually invisible. Here's a textureless playfield in Visual Pinball:


...which renders like that in Unity:


In most cases this shouldn't matter, since playfields are textured, and the kicker will be transparent.

A possible solution to implement the same behavior as Visual Pinball is to cut out the geometry of the playfield with a library like [triangle.net](https://github.com/garykac/triangle.net).