Reduce number of overlapping admin borders

This reduces the number of overlapping admin borders by rendering them in order
from high to low admin_level, and by giving all admin borders a white background
while using comp-op: darken.

Admin borders are currently grouped into three groups: level 1-4, level 5-8,
and level 9+. This changes only prevents overlap of admin borders within a
group (including overlap of borders of the same level). To prevent overlap of
borders within groups, it would be necessary to merge the groups, which would
lead to performance loss, and is therefore not included in this PR.

This partially solves #344.
Prevent overlapping admin borders of different layers

This prevents overlapping admin borders of different layers.
For performance reasons, the admin border layers are split into three groups
for low, middle and high zoom levels.
For each zoomlevel, all borders come from a single attachment, to handle
overlapping borders correctly.
Merge pull request #1107 from math1985/admin-prevent-overlap

Reduce number of overlapping admin borders