natural coastline should allow area

[ivanbranco]

"Natural coastline should be a way, not an area", is what iD says if you add natural=coastline to a small islet/island. But the Wiki allows this tag to be used on areas too.

[tyrasd]

This is a little bit tricky one to model correctly using the current schema. As the coastline features are "special" in OSM in in that the direction matters in which the coastline is drawn (land must be on the left and water on the right hand side of the way), it is important to show the small triangles which indicate where the respective sides are. iD does not render these indicators for areas. And since regular areas can be drawn in any winding order in iD, simply applying the coastline presets to them could lead to errors in the data.

In iD the following additional changes need to be implemented for coastlines:

• a validation routine in iD should check if the coastline is oriented properly -> https://github.com/openstreetmap/iD/issues/9292
• render triangle indicatos also for natural=coastline objects mapped as an area -> https://github.com/openstreetmap/iD/issues/9293