Add shape window

[serpilliere]

This branch adds the support of shaped windows.
Usage example:

    let mut builder =  sdl2::video::WindowBuilder::new(&video_subsystem, "title", width, height).set_shaped();

    let mut shape_surface = sdl2::surface::Surface::from_data(
        &mut shape_data,
        width,
        height,
        width * 4,
        sdl2::pixels::PixelFormatEnum::RGBA8888,
    )
    .map_err(|err| anyhow!("Cannot laod surface {}", err))?;

    let ret = window.set_window_shape(&mut shape_surface);

[Cobrand]

Thank you for the PR! I didn't know this feature was there, very interesting.

I have left a few comments, I think they're necessary for someone like me who didn't know what it was before looking at the source code.

[Cobrand]

Thanks!

[serpilliere]

Thanks for the merge @Cobrand !