bug: underground turning circles rendered on top of other features

[k-yle]

Related to #2558 and #4334

Expected behavior

Underground turning circles should be rendered in the same style as the tunnel they adjoin (or alternatively, not rendered at all).

Actual behavior

Unerground turning circles are rendered above feature at ground level, like buildings and pedestrian areas.

Screenshots with links illustrating the problem

Click each image

[imagico]

Thanks.

The problem is that turning circles - just like highway polygons (see #4544) - do not currently get rendered according to if they are part of a tunnel or bridge.

This could be fixed by adding turning-circle-bridges and turning-circle-tunnels layers in analogy to the linear road layers. But that would, just like for highway polygons, lead to quite significant growth of the road layer complexity overall. The alternative would be to integrate all of the different types of road geometries (linear, polygons and turning-circles) into unified layers (#2046).