Add RayMap::map_mut

[chompaa]

Objective

• Manually generate rays used for picking (partially resolves Support generating or redirecting rays for picking #16664).
• My use case is portals. I would like portal PointerHits to be propagated "through" the portal. This is of course doable by having manual backend implementations live locally, but having a mutable RayMap::map accessor can let these rays be processed next frame and handled by backends as usual.

Solution

• Add RayMap::map_mut.

Testing

• Ran CI.

Open Questions

• Can this make it into 0.15.1 (if uncontroversial)? I'm not overly familiar with Bevy's versioning :)
• Perhaps RayMap::push would be better, so that existing rays cannot be modified?
• Should we also have RayMap::iter_mut?

[chompaa]

I realised that mutating RayMap wasn't necessary for my use case, so I'm closing this PR (the mentioned issue has the same use case).

If any reader is curious what I did instead, I have my (poorly documented) impl here.