Function `grayscale_with_type_alpha`

fn grayscale_with_type_alpha<NewPixel, I: GenericImageView>(image: &I) -> ImageBuffer<NewPixel, Vec<<NewPixel as >::Subpixel>>
where
    NewPixel: Pixel + FromColor<LumaA<<<I as GenericImageView>::Pixel as Pixel>::Subpixel>>

Convert the supplied image to a grayscale image with the specified pixel type. Alpha channel is preserved.