Improve object layering when rendering data

[simonpoole]

We current render data in this order (bottom to top)

• multi-polygons and boundaries (ignoring layer tags)
• closed ways (taking layer tag in to account)
• ways (taking layer tag in to account)
• nodes
• turn restrictions

While just a rough approximation of correct layering this works quite well, however there are cases which can come out sub-optimal. For example landuse that is modeled with a closed ways that has buildings on top. There are multiple ways to attack this for example we could calculate area size and render smaller areas on top.