Coastline tweaks

[systemed]

This is a small number of tweaks to address remaining coastline geometry issues as per #580:

• Always run boost::geometry::correct before simplify_combine
• Replace obsolete BOOST_GEOMETRY_INCLUDE_SELF_TURNS with BOOST_GEOMETRY_NO_ROBUSTNESS (which should be standard now anyway)
• Add area filtering to the coastline layer below z12
• Area filtering now looks at the size of each outer (not just the total size) as well as that of the inners
• Remove special bay handling (#398)