img: stop ImageMagick trying to be clever if filenames contain a colon

img: stop ImageMagick trying to be clever if filenames contain a colon

$im->Read() takes a filename-like argument with several sets of special
syntax. Most of the possible metacharacters are escaped by the
default `wiki_file_chars` (and in any case not particularly disruptive),
but the colon ":" is not.

It seems the way to force ImageMagick to treat colons within the
filename as literal is to prepend a colon, so do that.