MaterialPipelineKey should have public fields

[Shatur]

In 0.6 MaterialPipeline used (MeshPipelineKey, M::Key) tuple as a key, where M is a custom material. But in 0.7 after #3959 it started using MaterialPipelineKey struct which is defined in the following way:

#[derive(Eq, PartialEq, Clone, Hash)]
pub struct MaterialPipelineKey<T> {
    mesh_key: MeshPipelineKey,
    material_key: T,
}

It look nicer, but all fields are private, so I can't call specialize in my user code for such materials (because the function accepts a key).

Here is how a real project uses it for overlay.
And here is my PR where I ported it. But it doesn't compile since in the mentioned place above I have to create MaterialPipelineKey (it compiles if I comment it out).

[superdump]

It looks like the fields were private as MaterialPipelineKey only used in MaterialPlugin. In the bevy-hikari case, it does not use MaterialPlugin because MaterialPlugin only queues meshes with the specialized materials to the opaque / alpha mask / transparent render phases, and bevy-hikari has its own phases to queue to. So it uses the SpecializedMaterial abstraction, but not the MaterialPlugin and then implements its own queuing. This seems like a shortcoming of the API at the moment. Making the fields public does seem like a quick way to regain the flexibility that was there in 0.6 without requiring significant code duplication or significant design effort to make the material plugin handle specification of the phase(s) to support.

[Shatur]

Fixed by #4508.