Restriction on the name of frame file

First of all, thank you so much for the great work on SAM 2.

I have a question regarding the name of frame file. It seems that the frame file must be named in number such as "00001.png" such that the function `load_video_frames` can work.

Here is part of the code in `sam2/utils/misc.py` at line 190

```
frame_names.sort(key=lambda p: int(os.path.splitext(p)[0]))
```

Will it be better if we can design our own sorting function? It is not very convenient that we need to rename the file as the filename may contain information sometimes.

Thank you.