Store image size in json file when exporting vector lay

[muellerflorian]

Kaibu allows to create annotations with different geometries. These annotations can be exported as a (geo)json file. I would be nice to have the actual image size in this file. This would allow to use these annotations to directly create image masks with the same size as the original image.

In order to follow the standards of geojson, this could be stored as bbox.

More details are here: https://tools.ietf.org/html/rfc7946#section-5