Added link wrapper, copied from base MediaTransformOutput class

A few people (myself included) wanted svg images to provide links just like other image media; I copied over the relevant bit of code from the base MediaTransformOutput class (https://doc.wikimedia.org/mediawiki-core/master/php/html/MediaTransformOutput_8php_source.html) to add that functionality. It seems to work fine in my application, but I haven't tested extensively -- ymmv.