fix: TypeError: 'float' object cannot be interpreted as an integer.

Cast frame duration as an int to avoid TypeError

```
Traceback (most recent call last):
  File "/workspace/backend/.venv/lib/python3.11/site-packages/imageio/core/v3_plugin_api.py", line 370, in __del__
    self.close()
File "/workspace/backend/.venv/lib/python3.11/site-packages/imageio/plugins/pillow.py", line 144, in close
    self._flush_writer()
  File "/workspace/backend/.venv/lib/python3.11/site-packages/imageio/plugins/pillow.py", line 485, in _flush_writer
    primary_image.save(self._request.get_file(), **self.save_args)
  File "/workspace/backend/.venv/lib/python3.11/site-packages/PIL/Image.py", line 2596, in save
    save_handler(self, fp, filename)
  File "/workspace/backend/.venv/lib/python3.11/site-packages/pillow_avif/AvifImagePlugin.py", line 117, in _save_all
    _save(im, fp, filename, save_all=True)
  File "/workspace/backend/.venv/lib/python3.11/site-packages/pillow_avif/AvifImagePlugin.py", line 252, in _save
    enc.add(
```

Author: BlackSmith

src/pillow_avif/AvifImagePlugin.py

@@ -287,7 +287,7 @@ def _save(im, fp, filename, save_all=False):
                 # Append the frame to the animation encoder
                 enc.add(
                     frame.tobytes("raw", rawmode),
-                    frame_duration,
+                    int(frame_duration),
                     frame.size[0],
                     frame.size[1],
                     rawmode,