How to supply a custom sampler to a texture?#60

At the moment the only way of creating a `Texture` with your own sampler (e.g. for nearest mag filtering) is to call `Texture::from_raw_parts`. However, to create a binding group we need to supply a shared reference to the texture layout which is currently a private field in the `Renderer` struct. The way I'm getting around it at the moment is to copy-paste the texture layout code from `Renderer::new`, but a better approach would be either for `Renderer` to have a function that returns the texture layout as a shared reference, or for `Texture` to have a `new` method that takes a `SamplerDescriptor`.