Render nature-reserve and theme park overlays below text and icon layers

This resolves #1046.

This resolves #1050
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.
clean up simple shape PNGs
replace simple non-SJJB PNG icons with SVG
Increase minimum font size for islands to 10pt

This is in line with the increased minimum font size for landcover.
Order building names, addr:housename, and addr:housenumber by area size

This resolves #1112
remove background color from polygon-pattern files
Decrease polygon-gamma

Decrease polygon-gamma and polygon-pattern-gamma to 0.3.
This resolves #672.
Don't render halo in roads in tunnels

This resolves #1089
Merge pull request #1129 from math1985/road-halo-tunnel

Don't render halo in roads in tunnels
Merge branch 'landuse-overlay-order' of https://github.com/math1985/openstreetmap-carto into math1985-landuse-overlay-order

Conflicts:
	project.mml
	project.yaml
Merge branch 'math1985-landuse-overlay-order'
Merge pull request #1117 from math1985/decrease-polygon-gamma

Decrease polygon-gamma
Merge pull request #1116 from math1985/building-text-sort

Order building names, addr:housename, and addr:housenumber by area size
Merge pull request #1115 from math1985/island-font-size

Increase minimum font size for islands to 10pt
Merge branch 'pattern-colors' of https://github.com/nebulon42/openstreetmap-carto into nebulon42-pattern-colors

Conflicts:
	landcover.mss
Merge branch 'nebulon42-pattern-colors'
Merge pull request #1113 from nebulon42/simple-svg-symbols

symbols: replace simple non-SJJB PNG icons with SVG
Merge pull request #1108 from nebulon42/png-cleanup

symbols: clean up simple shape PNGs