code: function (level, x, y) {
              var context = null;
              if (level >= this.minLevel && level <= this.maxLevel) {
                  context = this.levels[level].context2D;
              }
              return context;
          }
docs: Retrieves a tile context 2D
  @function
  @param {Number} level Level of the tile
  @param {Number} x x coordinate of the tile
  @param {Number} y y coordinate of the tile