Merge scattered meshes for performance
Removed the Corner method of random terrain generation
Added the ability to set a maximum slope on which a mesh can be placed by THREE.Terrain.ScatterMeshes;
Exposed the `stretch` option to THREE.Terrain;
Changed THREE.Terrain.Clamp() parameters so the `stretch` parameter is now part of the `options` map;
Added PerlinLayers and SimplexLayers terrain generation methods;
Renamed `perlinScale` to `frequency`;
Renamed `granularity` to `amplitude` for THREE.Terrain.MultiPass;
Fixed THREE.Terrain.MultiPass and THREE.Terrain.ScatterHelper are now thread-safe;
Fixed THREE.Terrain.MultiPass amplitude calculations