Add imagery for islands between Korea and Japan, claimed by both

The ownership of the islands in question is disputed so I have not added a country code.

The imagery is 1m/px panchromatic orbview-3 imagery from the USGS (public domain)

It has been lanczos resampled to z17 web mercator (95.1 cm/px) and histogram streched and contrast adjusted. It is hosted on errol.

See pnorman/mapproxy-config@5a0b193c83f0245115ce93ae0c9b9df2896d08b8 .. pnorman/mapproxy-config@2151a591058aed6ab8a2e76a69a6487815d3ae74