bug: ImageExporter should delete temp dir

[Dirk--]

ImageExporter should delete temp dir:
If temp dir [mbtiles_name] exists, it will not be overwriten with new content thus. giving an image with old content

[leplatrem]

Good catch!

Currently this part of the project is not used internally at makina corpus, and thus not maintained actively.

Although, if you have time to submit a pull request we would happy to review it!