计算坐标范围
重载 DxfEntity .
被 DxfBlock, DxfInsert, DxfHatch, DxfDimension, DxfDimAligned, DxfDimAngular, DxfDimDiametric, DxfDimLinear, DxfDimRadial, DxfPolyline , 以及 DxfSpline 重载.