This doesn't address stacking multipolygons, or interleaving
areas and multipolygons. More and more, I'm thinking we should
just merge the two into a unified "area" geometry.
An entity that is a member of a relation will have the classes
`member`, `member-role-<member role>`, and `member-type-<relation type>`.
The first use of these classes is to avoid filling multipolygon
member areas.
Now that Graph#parentWays is cached, the specialized Graph#parentStructure
method is no longer necessary.
This commit also demonstrates that it's relatively easy to write specs for
rendering now!