Proper camera logic, and new player movement limitations

[64kramsystem]

Closes the first task of #29; the camera has now a proper logic, typical of beat 'em ups.

Boss-related logic is not yet implemented, since there are no bosses yet 😆

[64kramsystem]

Ok, this is ready for review etc. Note that this logic has severe limitations. This is intentional; fully implementing the new camera logic requires extensive changes, which IMO should not be applied in a single PR.

[64kramsystem]

Addressed all of them. Generalized the forward (right) limit to any number of players.

Note that the limitation on the backward (left) behavior is not relative to the rightmost player position, instead, it's based on the boundary. This can make the distance grow more than wanted. This is intended, since the limitation logic will need to be updated along with the new camera logic.

[64kramsystem]

Extra note: I'm using a feature of the latest stable Rust (f32#total_cmp).

[64kramsystem]

Since nobody was looking, I've added the new camera commits 😬 😂 This now fully closes the first task of #29.

[64kramsystem]

Re-requested review, due to new commits with the new camera logic.