Annulus sampling

[mweatherley]

Objective

Add random sampling for the Annulus primitive. This is part of ongoing work to bring the various bevy_math primitives to feature parity.

Solution

Annulus implements ShapeSample. Boundary sampling is implemented in the obvious way, and interior sampling works exactly as in the implementation for Circle, using the fact that the square of the radius should be taken uniformly from between r^2 and R^2, where r and R are the inner and outer radii respectively.

Testing

I generated a bunch of random points and rendered them. Here's 1000 points on the interior of the default annulus:

This looks kind of weird around the edges, but I verified that they're all actually inside the annulus, so I assume it has to do with the fact that the rendered circles have some radius.