Add force-conversion option to skip imlib2 image loading and directly try to convert the image instead

[lazypingu]

This PR adds the option force-conversion which skips the regular imlib2 image loading before a conversion will be attempted.

This might be helpful in cases where imlib2 is able to load the image but e.g. just a tiny thumbnail. This would prevent a further conversion, since the image was loaded successfully.

I faced this with .NEF raw files of a Nikon Z7 II camera.

With force-conversion enabled, feh will directly try to convert with dcraw or image-magick.

Since force-conversion only makes sense with enabled image-conversion, the latter option will be checked and set to zero if missing or negative.