albedo_backend: Improve Primitive API

[DavidPeicho]

• Make AttributeSlice a generic over the underlying slice:

let index = primitive.atribute("POSITION");
if let Some(slice) = primitive.attribute_slice_f32(index) {
    for v in slice {
        v = v + glam::Vec3(1.0, 1.0, 1.0);
    }
}

• Create interleaved vector from Vec<V: Pod>
  • This needs to be unsafe to reinterpret the vector