Support polygon bounding boxes in mapsources.

[grst]

For example in the outdooractive germany map we end up having white tiles, as Outdooractive doesn't provide tiles outside Germany. By now, we can only define the bounding box as a rectangle which is not precise enough to capture the boundaries of the map.

Use something like counterclockwise points in the xml:

<region>
    <point><lat>...</lat><lon>...</lon></point>
    <point>...</point>
    ...
</region>