Show inverted polygon for allowed zone

[nrenner]

Out of interest I experimented with adding an inverted polygon for the limited area, as a potential addition to #347.

• shows the outside area in a darker shade
• shows a not-allowed cursor
• stops clicks on the polygon and thus prevents adding markers outside the area - but cannot avoid dragging markers out

@bagage what do you think, would that be a useful addition (I don't mind if not)?

[bagage]

Very nice enhancement @nrenner, it isn't much more code and the result looks way better like that!

Do you think we could do a release for that to be available on brouter.de main instance? It would be a really great enhancement for French users :).

[nrenner]

Do you think we could do a release for that to be available on brouter.de main instance?

Done.