Support disabling drawing the clip plane

[gkjohnson]

Right now, when the clippedRenderer script is disabled the material is still rendered with discarded pixels. We need some way to disable the clip plane in the material, then.

A shader keyword might work, but then we need to create a new material. It might be best to do something like make a 0, 0, 0 disable the plane, make another material property to disable it, or just set the plane distance to be really large.