Function blurred_cover 

pub fn blurred_cover(
    image: &ImageData,
    rect: Rect,
    blur_px: f32,
) -> Option<(ImageData, Affine)>

image covering rect as a desktop paints a wallpaper (scaled to cover it, centred), blurred by blur_px logical pixels: the blurred picture and the transform that places it over rect. Made once per wallpaper, size and radius on this thread. None for an image whose pixels are not in memory.