Support for WebP thumbnails via --picture

[vcalv]

The --picture options allows either a specification as

[TYPE]|[MEDIA-TYPE]|[DESCRIPTION]|[WIDTHxHEIGHTxDEPTH[/COLORS]]|FILENAME

Or just a filename.WEBP

I can understand that when specifying just the filename, only jpeg, png and gif are supported, since all the parameters (dimensions, depth and media-type) have to be inferred.

The thing is that even when specifying all the parameters for a webp image, it's still not supported since part of the code verifies whether the image is one of the recognized formats.

This seems senseless to me and I would like to be able to specify whatever parameters I want via the full spec.