Added optional serde derives for rendering types.

[mintlu8]

Objective

Certain types in bevy_render, bevy_pbr, and bevy_sprite have trivial serde implementations but currently doesn't. Adding them can benefit developers of prefab crates.

Solution

Added serde implementations for these types.

Testing

Cargo build with the serialize feature.

[janhohenheim]

Some notes:

• The vibe regarding this in general is that rendering types should not be serialized. Hence why e.g. SerializedMesh exists.
• Still, I think it's alright to implement them on these types. I certainly wouldn't veto it, though I wonder if using a serliaez impl on these types is pointing to underlying issues in the prefab impl
• What does ExtendedMaterial do when its generics are not Serialize or Deserialize?
• If we merge this, I would want to also conditionally #[cfg_attr(feature="serialize", reflect(Serialize, Deserialize))

[mintlu8]

Some notes:

• The vibe regarding this in general is that rendering types should not be serialized. Hence why e.g. SerializedMesh exists.

Rendering types cannot be serialized because Handle exists. Types that doesn't contain Handle can be easily serialized. (We could do some thread local shenanigans but thats way out of scope for this.)

• Still, I think it's alright to implement them on these types. I certainly wouldn't veto it, though I wonder if using a serliaez impl on these types is pointing to underlying issues in the prefab impl

It's more if the prefab crate want to express something like AlphaMode it doesn't have to add a custom type.

• What does ExtendedMaterial do when its generics are not Serialize or Deserialize?
• If we merge this, I would want to also conditionally #[cfg_attr(feature="serialize", reflect(Serialize, Deserialize))

Added Reflect impls, although for ExtendedMaterial I'm not sure since the type bounds do not match.

[janhohenheim]

It's less about Handle and more about not wanting people to depend on the format of various render types like Mesh.
It's probably fine in this case? Pinging @tychedelia

[tychedelia]

Having a conversation with some of the other rendering folks about how we want to approach this as it's coming up a lot recently.

[IceSentry]

@tychedelia do you remember if anything conclusion was reached?