tiling: Don't completely empty buffer in animation

Previously, this would lead to flickering if the decoder was too slow.
Now, the last frame will be shown until a new one is available.

Closes #382

build-aux/flatpak/org.gnome.Loupe.Devel.json

@@ -159,7 +159,7 @@
                 {
                     "type": "git",
                     "url": "https://gitlab.gnome.org/sophie-h/glycin.git",
-                    "commit": "4b0759839322fad9277c7f823438abe4937e4027"
+                    "commit": "9701eb7ac36cd85ce75241bf50dfe325b50f54c9"
                 }
             ]
         },

src/decoder/tiling.rs

@@ -648,10 +648,11 @@ impl SharedFrameBuffer {
     /// Return true if the next frame should be shown and removes the outdated
     /// frame
     pub fn frame_timeout(&self, elapsed: Duration) -> bool {
-        if self
-            .buffer
-            .load()
-            .images
+        let images = &self.buffer.load().images;
+
+        // Only move to next image if there is one buffered
+        if images.len() > 1
+            && images
                 .front()
                 .is_some_and(|next_frame| elapsed >= next_frame.delay)
         {