Mgv5 get ground level at point function for locating spawn of new player

This uses non-eased 3D noise to approximate the eased 3D noise used in mapgen, then adds 8 nodes as a safety margin to avoid burying the player. In roughly 50 tests the player was buried in a wall once, being dropped into deep water is common. When we have a 'get eased noise at point' function this code will be made exact but for the moment this works fairly well and is urgently needed for player spawn.