Raycasting for primitives.

[Olle-Lukowski]

I have recently started working on https://github.com/Olle-Lukowski/bevy_contrib_raycast to allow for raycasts on both 2d and 3d primitives. I thought I would open up a tracking issue here, so more people can review / help out with the code.

Here is a checklist of what needs to be done:

• [x] Implement RayCast2d for all 2D primitives (quick and dirty, to get a working example)
• [ ] Implement RayCast3d for all 3D primitives (quick and dirty, to get a working example)
• [ ] Optimize and clean up 2D impls.
• [ ] Optimize and clean up 3D impls.
• [ ] Add tests.
• [ ] Write some docs.
• [ ] Get reviews, fix any issues.

[janhohenheim]

@alice-i-cecile should we close this in favor of #12365 ?

[alice-i-cecile]

IMO no: this issue is more specific and still desired. The other issue is largely complete though.

[BenjaminBrienen]

I have implemented for cuboid with tests https://github.com/Olle-Lukowski/bevy_contrib_raycast/pull/2

[Jondolf]

Implemented everything in #15724 :)

I'll most likely split it into several smaller PRs over the 0.16 cycle.