Implement 3d texture as assets

[sim-the-bean]

This is a simple pr that implements 3d textures as a mostly-copy of 2d textures.

One big, potentially breaking change is that now RenderResource requires an implementation of fn texture3d, which works just like fn texture. The derive macro has been updated to support this.

I haven't tested this yet properly, so I'm making it a draft until I've verified that this works.

[tsal]

Would this be better served as a new Trait that extends the RenderResource trait - something like:

pub trait RenderResource3d: RenderResource  {
    fn texture3d(&self) -> Option<Handle<Texture3D>>;
}

I'm still a bit new to Rust, but if I'm understanding traits, this kind of makes sense if we want to write systems specific to 3d textures vs 2d.

[cart]

Closed in favor of #903