Allow eww to display exotic images like webp

Allow eww to display exotic images like webp

* lisp/image.el (image-type): Allow passing in the image type.
(create-image): Make conversion work with data in addition to files.

* lisp/image/image-converter.el (image-convert-p): Allow taking
working on data in addition to files (bug#38036).
(image-convert): Ditto.
(image-converter--convert): Extend signature to say whether we're
getting a file or data.
(image-converter--convert-magick): Convert data.
(image-converter--convert): Ditto.