Feature enhancement to be able to add a watermark image to tiles at the moment they are stored to cache.

now accepts a /path/to/file.png child.

the supplied image should have the exact same size as the size of the tiles configured for the tileset.

note that the watermark image is decoded to memory when the configuration file is parsed, so the watermark image will not be reloaded if it is changed until the server is restarted.