Function `grayscale_alpha`

fn grayscale_alpha<I: GenericImageView>(image: &I) -> crate::ImageBuffer<crate::color::LumaA<<<I as GenericImageView>::Pixel as Pixel>::Subpixel>, Vec<<<I as GenericImageView>::Pixel as Pixel>::Subpixel>>

Convert the supplied image to grayscale. Alpha channel is preserved.