[Mapcss] Layering model enhancements

[Komяpa]

Hi everyone,

Thinking of casings/polygons problem.

Imagine a linear road that finishes as asphalted surface inside a
forest. Let the road be highway=service; the asphalted surface -
highway=service area=yes, and forest - landuse=forest.

Currently, we draw:
 - forest as green;
 - highway=service area=yes polygon as white
 - casings for both highways. The casing for linear service overlaps
with white area's polygon.
 - linear fill for highway.

That leads to rendering artifacts. We can omit them by rendering first
casings, then polygons - but that leads to roads being casingsless in
forests.

What I propose:
 - add background-*, same as fill-*, rendered before casings;
 - render fill-* in the same loop with lines.

Any objections / better ideas?

-- 
Darafei "Komяpa" Praliaskouski
OSM BY Team
xmpp:me at komzpa.net mailto:me at komzpa.net