decoder: Fix animations with variable frame length

Before, the length of the first frame was used for all frames.

Part-of: <https://gitlab.gnome.org/GNOME/loupe/-/merge_requests/300>

src/decoder/formats/glycin_proxy.rs

@@ -106,7 +106,7 @@ impl Glycin {
                                 texture: frame.texture,
                             };
 
-                            tiles.push_frame(tile, dimensions, delay);
+                            tiles.push_frame(tile, dimensions, frame.delay.unwrap_or(delay));
 
                             if tiles.n_frames() >= FRAME_BUFFER {
                                 break;

src/decoder/tiling.rs

@@ -661,7 +661,7 @@ impl FrameBufferExt for ArcSwap<FrameBuffer> {
         if self
             .load()
             .images
-            .get(1)
+            .front()
             .is_some_and(|next_frame| elapsed >= next_frame.delay)
         {
             self.rcu(|tiling_store| {