Fixed the problem of some faces not showing, it was solved by setting
ContextSettings.depthbits to 24.

There is still something wrong with the basic pyramid manually defined,
sometimes the bottom and front sides arent showing. However, I am not
going to focus on that right now, because the application seems to be
able to load and render the meshes from Blender well enough to proceed.

The next goal is to fix the mesh loading algorithm so that it is able to
handle untriangulated meshes and meshes that contain information about
normals.