saffron export stuff

public class SB3Exporter {
    public static void exportSprite3 (ZipOutputStream zos, SaffronCanvas c) {
        BufferedImage[] images = new BufferedImage[lastFrame(c)];
        CompletableFuture<Void>[] futures = new CompletableFuture<Void>[images.length];
        for (int i = 0; i < images.length; i++)
            futures[i] = CompletableFuture.supplyAsync(() -> c.drawForOutput(i)).thenAccept((b) -> images[i] = b);
        CompletableFuture.allOf(futures).thenAccept((v) -> {
            for (BufferedImage img : images) {
                zos.putNextEntry(new ZipEntry(getName(img)));
                ImageIO.write(img, "png", zos);
                zos.closeEntry();
            }
            zos.putNextEntry(new ZipEntry("sprite.json"));
            zos.write(getContent(images, c).getBytes());
            zos.closeEntry();
        });
    }