Some Feature's properties are handled, such as the type of the
element, its height...

A new MultiMeshes 3d object is added, to 'pack' several meshes into
one single OpenGL buffer. It is needed to fast-render the huge numbers
of buildings that can be generated from a GeoJson file...

A MeshContainer is also added. It internally handles a vector of
MultiMeshes, automatically allocating a new MultiMeshes if needed when
a new mesh is added to the container.

Note: this commit is the result of a work from Paul-Elian Tabarant
and Guillaume Dauster. The individual commits comes from two separate
git repos, hence this 'merged' single commit.