[Mapcss] Background and fill for polygons

[Komяpa]

Hi all.

While developing the stylesheet I bumped (again) into the fact that
we've got two types of polygons:
 - background ones, like forests;
 - foreground ones, like highways.

When rendering, you usually want your casing to lie over the forest,
but under polygonal road, for example. Or have your casing over the
place=city polygon, and under building=yes if it crosses it (but still
a road over the building).

I think it could be fixed easily by adding background-* to fill-*.

Anyway, we've got "background lines" aka casings already, so I don't
see why we shouldn't introduce "background fills".

If a renderer doesn't want to support it, it could either just drop
it, or process the same way as fill-*.

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