Unloadable image detection is slow with Imlib2 ≥ 1.6

When I run `feh Folder_name` and iterate over the images, feh is stuck on large .mp4 files. It logs:
```
feh WARNING: *.mp4 - No Imlib2 loader for that file format
...
```
And feh may just not respond to any commands.
Is it possible to add command line arguments:
- list of extensions that can be opened
- list of extensions that can't be opened
- file size limit

All optional