This is executed if the camera has moved into a new CameraCell and will load in
the new TerrainQuad tiles to be children of this TerrainGrid parent.
It will first check the LRU cache to see if the terrain tile is already there,
if it is not there, it will load it in and then cache that tile.
The terrain tiles get added to the quad tree back on the OGL thread using the
attachQuadAt() method. It also resets any cached values in TerrainQuad (such as