Faceted Sphere Primitive

[klarh]

In case anyone has a need for these, it should be relatively straightforward to add them, especially now that we have the infrastructure in place to automatically build array uniforms in the vispy backend.

Attached here is an example from an old notebook implementing the shaders.

Suggestions for capabilities/attributes:

• Per-particle colors (that color the sphere parts)
• Configurable facet plane equations
• Configurable facet colors (if fewer facet colors than facet equations are given, use the per-particle color)
• A facet_color_mixture attribute to mix between per-particle colors and per-facet colors
• Positions, orientations, diameters (would we want to just make the diameters fixed for the entire primitive, or allow per-shape diameters?)