Improve marsh/wetland/mud rendering

* Render marsh and wetland from z10 (resolves #899)
* Render labels on marsh, wetland and mud (resolves #935)
Add new marsh/wetland icon

Thanks to @imagico for proposing the icon:
https://github.com/gravitystorm/openstreetmap-carto/pull/1042#issuecomment-5
Merge branch 'mud-marsh-wetland-names' of https://github.com/math1985/openstreetmap-carto into math1985-mud-marsh-wetland-names
Merge branch 'math1985-mud-marsh-wetland-names'