Reduce number of overlapping admin borders

This prevents 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 0-4, level 5-8, and level 9+. Overlap between these groups is avoided, without compromising performance, by creating one attachment for admin_levels 0-4 for low zoom levels, one for admin_levels 0-8 for mid zoom levels, and one for all admin_levels for all zoom levels.

This solves #344.