[Feature Request] Support loading of buildings like OSM Buildings

[florianf]

It would be great, if a feature like https://github.com/kekscom/osmbuildings could be integrated a layertype in OpenWebGlobe.

The implementation of OSM Buildings looks like:

1. Fetch the buildings 2d footprints and height of twice the current map extent from a database
2. Extrapolate the 2d poylgon to a 3d shape. Triangulation and mesh creation should happen automatically.
3. Render the resulting shapes.

I really think this could be useful for OpenWebGlobe, because most building data is footprints of buildings including height.

The resulting mechanism could easly be extended to support texturing of buildings (specify a texture for every wall and one for the roof).

The current possibilities of importing of json files for buildings are IMHO too static for larger sets of buildings.