replaced TileSpec.DEFAULT_MESH_CELL_SIZE by
RenderParameters.DEFAULT_MESH_CELL_SIZE because the values should be
sunchronized and the main focus lies on rendering

TODO to self: the parameter is also used to derive bounding boxes without
eventually rendering tiles.  However, these bounding boxes are usually
considered to represent the area a tile covers if it would be rendered.
Finish this thought some time near future.