Get filename from lists.txt correctly

my `lists.txt` has lines that look like this:

```
001.jpg;1080;1920;4.1;0;540;0;4.1;960;0;0;1
002.jpg;1080;1920;4.1;0;540;0;4.1;960;0;0;1
...
```

so need to split on the first semi-colon to get the filename correctly. Note: `find_first_of(';')` returns the length of the string if it doesn't find a semi-colon, so this should work even if there is some other file format I'm not seeing without the extra metadata.