Width and height properties for Documents with image mediaTypes

[jfietkau]

Mastodon uses Documents with image mediaTypes in the attachments list to model image attachments. These documents supply width and height properties with integer values to let clients know about the expected render size of the image. These properties are not part of the ActivityStreams vocabulary definition of Document nor a specified extension by Mastodon, so they're AIUI straight-up out-of-spec.

Nonetheless, it would be useful if Fedify could serialize width and height properties as part of the JSON-LD if they have been added to a Document object, to make it possible for clients to predict image dimensions before the attachments are downloaded.