Import of background images from an image server

[imi]

Import images from a tile server and display them as background layer.

The server should be configurable (and speak WMS).

To not spam the server, JOSM should not just reload everything at every display move but should cache tiles on disk and reuse this data.

[imi]

This will probably done by Nick as an plugin.

[Tweety]

Maybe you can use tiles with fixed size. This way some other applications might be able to use these on you local machine too. If The tiles happen to be 1280x1024 we can try to share the Landsat-tile folder with gpsdrive.