Planar projection (perspective/orthographic combination projection)

[thatcomputerguy0101]

This adds a new type, PlanarFOV, and a new function planar to create a projection matrix for which the view space is projected to a plane at zero depth, currently two units high. This allows the focal point to move freely according to the specified vertical field of view, making for a continuous transition from positive FOV to zero or even negative FOV.

I'm unsure of using the name PlanarFOV for this, which was derived from how it keeps a plane's transformation fixed as the FOV is adjusted. Additionally, it may be appropriate to add a height parameter to customize the height of the fixed plane instead of fixing it to extend one unit in each direction.

If desired, I can add some test cases to verify the boundary constraints that this projection was derived from.

[aloucks]

Why was the value two selected for the height? It seems arbitrary. Could this be configurable?

With that said, cgmath hasn't had a release in nearly 4 years and doesn't currently have a full time maintainer. You might try starting a discussion on the glam library repo and see if there is interest in adding this functionality .

[thatcomputerguy0101]

A height of two units was where the math of a symmetric view worked out to be the simplest, with one unit above the origin and one unit below the origin. However, it isn't too complicated to make the height configurable, so that can be changed.

I was making this PR upstream of the crate that I'm directly using (three-d), so I hadn't considered the space of equivalent math crates.