Order amenity_lines by layer

[johsin18]

This way, we respect the layer tag for water slides, and it should not harm for fords.

Fixes #3413

Changes proposed in this pull request:
Order amenity_lines by layer

Test rendering with links to the example places:
https://www.openstreetmap.org/#map=19/48.51073/9.03904

Before

After

[kocio-pl]

Looks very good and simple, should not do any harm for both current features.

[kocio-pl]

Interesting - my local rendering from the current version works as expected without this fix. Could anyone else check it?

I guess however that sorting should be safe anyway. At least it does not break my local rendering.

[johsin18]

Without the explicit ordering, the DB will return the segments in an undefined order. So with only two segments, you still have a 50% chance to get the right result. With 3 segments, the probablity goes down to 16%, however, it might not really be random, but depend on the points in time the segments were added.
Try this one with 3 segments:
https://www.openstreetmap.org/#map=19/48.52361/8.08900&layers=N
Or change the code to "ORDER BY COALESCE(layer,0) DESC" to break it for sure.

[kocio-pl]

Thanks! With DESC it was inverted, but without it it worked as it should. I don't know why my setup works different than OSMF servers, but that should be OK.