Images with alpha channel not rendered

[robinkrahl]

Images with an alpha channel are not rendered by printpdf. See for example this modification of the images.rs example:

extern crate printpdf;

use printpdf::*;
use image::{GenericImage, GenericImageView};
use std::fs::File;
use std::io::BufWriter;

fn main() {
    let (doc, page1, layer1) = PdfDocument::new("printpdf graphics test", Mm(20.0), Mm(20.0), "Layer 1");

    let (mut page, mut layer) = (page1, layer1);
    let image_constructors: &[(&str, fn(u32, u32) -> image::DynamicImage)] = &[
        ("bgr8", image::DynamicImage::new_bgr8),
        ("bgra8", image::DynamicImage::new_bgra8),
        ("luma16", image::DynamicImage::new_luma16),
        ("luma8", image::DynamicImage::new_luma8),
        ("luma_a16", image::DynamicImage::new_luma_a16),
        ("luma_a8", image::DynamicImage::new_luma_a8),
        ("rgb16", image::DynamicImage::new_rgb16),
        ("rgb8", image::DynamicImage::new_rgb8),
        ("rgba16", image::DynamicImage::new_rgba16),
        ("rgba8", image::DynamicImage::new_rgba8),
    ];
    let red = image::Rgba([255, 0, 0, 255]);
    let font = doc.add_builtin_font(BuiltinFont::Helvetica).unwrap();

    for (i, (label, image_constructor)) in image_constructors.into_iter().enumerate() {
        if i > 0 {
            let (new_page, new_layer) = doc.add_page(Mm(20.0), Mm(20.0), "Layer 1");
            page = new_page;
            layer = new_layer;
        }

        let mut image = image_constructor(100, 100);
        for x in 0..image.width() {
            for y in 0..image.height() {
                image.put_pixel(x, y, red);
            }
        }

        let image = Image::from_dynamic_image(&image);

        let current_layer = doc.get_page(page).get_layer(layer);
        current_layer.use_text(*label, 12.0, Mm(0.0), Mm(10.0), &font);
        image.add_to_layer(current_layer, None, None, None, None, None, None);
    }

    doc.save(&mut BufWriter::new(File::create("test_image.pdf").unwrap())).unwrap();
}

Only the images without an alpha channel are present in the generated PDF file:
test_image.pdf

(This was initially reported against genpdf-rs).

[robinkrahl]

Sorry, I just realized that this is a duplicate of #84.