Unpaved rendering for turning circles / mini-roundabouts

[dch0ph]

Closes #4770.

Changes proposed in this pull request:

• Builds on dormant PR Unpaved rendering for turning circles #4770 from @sommerluk to add unpaved pattern to turning circle/loops and mini roundabouts
• Simplification of SQL selection to use highway z order rather than bespoke prioritisation table

Test examples are difficult to find, and subtle issues associated with mini roundabouts are best illustrated with a pseudo-legend:

Notes:

• As in Unpaved rendering for turning circles #4770, an artificial line of negligible length is used to work around mapnik limitations (the line patterns used for unpaved rendering cannot be applied to nodes)
• The surface of a mini-roundabout / turning circle is not generally specified, and is instead determined from the way(s) involved. As in Unpaved rendering for turning circles #4770, the paved/rendering at mini roundabouts is determined by the highest priority road (1st and 2nd examples above). In the case of equal priority, paved (no pattern) is preferred (third example above).

For discussion:

• We could introduce a CTE so that the MSS for paved receives a normal node rather than an artificial way. This may be slightly less hacky.
• It would make sense to use functions to determine the paved status, rather than duplicate code between SQL queries.

[imagico]

This looks fairly good - but i think two technical changes would be advisable:

• layers should get geometry: linestring
• the int_surface generation could be moved into a subquery and can then be used for the ORDER BY without repeating the value list.

I am also wondering if - since you generate a line geometry for all turning circles anyway it might make sense to use line symbolizers also for the paved version instead of marker symbolizers. This would make the code slightly shorter (no separate marker-width/marker-height required).

[dch0ph]

Changes made.

There are minor unexpected changes in switching all the rendering to line based in the PDF output above (slightly thicker casing on roundabout in 2nd example), but this is confined to the PDF output, and there are no changes in the raster output. A couple of examples at Z16:

The simpler and more consistent code is an advantage on balance.