code: public SceneBlock getBlock (int tx, int ty)
      {
          int bx = MathUtil.floorDiv(tx, _metrics.blockwid);
          int by = MathUtil.floorDiv(ty, _metrics.blockhei);
          return _blocks.get(compose(bx, by));
      }
docs: Returns the resolved block that contains the specified tile coordinate or null if no block
  is resolved for that coordinate.