title: GTK4: Picture does not shrink
body: <p>I am trying to insert an image into a header bar in GTK4. The image should fit the height of the parent widget. In gtk4-rs I tried:</p>
  <pre><code>let image = gtk::Picture::builder()
          .hexpand(false)
          .vexpand(false)
          .halign(gtk::Align::Fill)
          .valign(gtk::Align::Fill)
          .keep_aspect_ratio(true)
          .can_shrink(true)
          .build();
  </code></pre>
  <p>Although <code>can_shrink(true)</code> is set, the image causes the header to grow. I also tried <code>.height_request(1).width_request(1)</code>, but that also had no effect.</p>
  <p>How can I make the image fit the header's size determined by the other widgets in the header?</p>