Implement trivial clone for WindowBuilder

[SolraBizna]

I'm writing a library that can attempt a few different methods of rendering into an SDL window. I wanted to have the library's user specify the window attributes they want, then pass that partially-complete WindowBuilder into my create_renderer function, which clones it for each renderer so that additional flags (e.g. .opengl() or .vulkan()) can be set before attempting to build the window in each case. Basically, I would like to be able to write:

pub fn create_renderer(video: &VideoSubsystem, builder: WindowBuilder)
    -> Result<Box<dyn Renderer>, somethingsomething...> {
    vulkan::create(video, builder.clone())
    .or_else(|| gl32::create(video, builder.clone())
    .or_else(|| gles::create(video, builder.clone())
    .or_else(|| soft::create(video, builder.clone())
}

But as WindowBuilder doesn't implement Clone, I must instead use a different pattern, creating a fresh new WindowBuilder each time.

I had a quick look and it seems that simply adding #[derive(Clone)] to WindowBuilder would meet my needs. I didn't see anything that looked like it would break.