Don't use Multi for processing VN cover images

I used to do this to avoid loading Image::Magick in each TUWF process,
decreasing memory usage, and lowering the blocking time by avoiding too
much processing. Memory isn't much of a problem nowadays, and processing
images is fast enough, too, so this complexity isn't necessary anymore.

(Character images and screenshots pending)
Don't use Multi for processing character images
Don't use Multi for processing screenshots

TODO: Get rid of the 'processing' flag and all the async loading of
screenshot data in the screenshot uploader.
Screenshot uploader: Load stuff synchronously + removed processing flag

All the async stuff isn't necessary now that images are processed
synchronously.