Add basic flat plane buoyancy demo using bounding boxes and grid raycasting

[jsBread]

Adds basic buoyancy on a flat plane by raycasting a grid defined by the bounding box of a shape. These ray collisions define a set of rectangular prisms that approximate the submerged volume and its submerged volumetric center. We attempt to improve the approximation by using the ray collision normals to scale the submerged volume of a given rectangular prism, but it's by no means perfect.

There's plenty of improvement to be had here, so this might just act as a decent basis for a buoyancy demo. I've left in the ability to plug in wave heights if someone wants to add in some deforming mesh or heightmap to act as water. Both linear and angular drag forces do an okay job, but could use improvement as well