albedo_backend: MeshAttribute

[DavidPeicho]

Create MeshAttribute struct:

pub struct VertexAttributesLayout {
    attributes: Vec<VertexAttribute>,
    data: Vec<u8>,
    names: Vec<String>,
    stride: u16,
}

Add a derive to automatically create attribute from vertex struct:

#[derive(AsVertexAttributesLayout)]
struct Vertex {
    position: [f32; 3];
    uv: [f32; 2];
}