More general slicing primitives

[nmwsharp]

trafficstars

Could implement lots of primitives beyond just slice "planes". The shader system is set up to support this, but some boilerplate setup and GUI/API decisions are needed.

Other primitives:

• slice sphere
• slice cube
• slice slab (keep stuff within a fixed width from a plane)

[the13fools]

stretch goal: Use heat method from boundary, and slice level sets of the integrated normalized field, useful for seeing what's going on as a function of distance from the surface.

stretch goal 2: CSG operations on sliced primitives, i.e. what if I want to select some bounding box, and then slice slabs at a time.