Planar projection camera

[thatcomputerguy0101]

This adds another camera projection mode planar for which the view space is projected to a plane at zero depth. The way the projection frustum is computed allows the FOV to be zero or negative, effectively combining the perspective and orthographic camera modes. However, since the focal point usually does not coincide with the camera position, this works best as an orbit/third-person camera. This projection mode is based on #40 to correctly scale during zoom inputs. The math for this projection was merged into cgmath in rustgd/cgmath#556, but is unlikely to see a release soon so it was also copied to the math file of this crate.

Zooming may be possible without #40, but the combination of height and camera position affecting zoom in perspective mode might not work as well as purely scaling the view transform.

[asny]

Looks good to me. What exactly do you need it for? 🤔

[thatcomputerguy0101]

I'm working on a program similar to https://hypercubing.xyz/hyperspeedcube/ but less 4d, where the focus is always on the same object at the same size. Therefore, when the FOV is adjusted, I wanted the puzzle to stay the same visual magnitude, and negative FOV was a free bonus of the math I ended on to allow that.