Importer crashes due to dereferencing deleted memory

First of all, thanks a lot for making this great library!

When importing some larger datasets I ran into some crashes.

This appears to be the cause:

In the call to gluTessVertex we pass a pointer to elements stored inside the _vertices and _indices vectors.
However, gluTessVertex does not appear to do any work until we call gluTessEndPolygon.
At this point, _vertices and _indices may have been re-allocated (after successive calls to TesselatorBase::addContour which pushes new elements to them, causing them to re-allocate to fit a new capacity)
The memory pointed to will be freed, and accessing it (in vertexDataCallback and combineCallback) will trigger undefined behaviour.

We can fix this by making sure the vectors containing the elements we send to GLU do not grow while GLU is processing them. That would be really simple if we knew before starting how many vertices we will need to add. Unfortunately I don't think we know that?

So here's my suggested solution:

Build the list of vertices and indices for all contours, before starting to process them.
- This will make sure calls to addContour will not happen while GLU is processing them.
Copy the index value directly to the void* data parameter, so that we can safely add new elements to _indices (from Tesselator::combineCallback) while GLU is working with them.
copy _vertices to a new temporary std::vector, and pass vertex pointers from this list instead, so we can safely add new elements to _vertices (from Tesselator::combineCallback) later.

It's confusing to explain, so I'll send you a PR :grin:

jklimke / libcitygml

Importer crashes due to dereferencing deleted memory #76