Ignore `building=*` in isometric mode

[enzet]

In isometric mode if the building has building:part=* tags, ignore outlines building=*, since they may hide all the details we try to draw. See Simple 3D Buildings.

Things to do:

1. Decide, whether or not building=* is an outline for a set of building:part=* elements.
2. Remove shapes for building=*.

Examples

Outline shape of Atomium is drawn under the actual structure:

map-machine render -c 50.894926/4.341521 -z 18 --buildings isometric