Hi, I'm manipulating the vertices of a ground mesh using the .updateMeshPositions() method and  now i want to get the height at a specific point (x,z) on the mesh with .getHeightAtCoordinates(). Unfortunately, this doesn't seem to work... Here is a playground example: http://www.babylonjs-playground.com/#1JXJVF#2   What am I doing wrong?  Thank you! BMA