计算任意多边形和矩形的交点
//the input rectClip must have been NormalizeRect().
//	CRect rectClip(m_ptnOrign, m_ptnPre);
//	rectClip.NormalizeRect();
void CheckPolygonClip(const vector &polygon, 
					  vector &vSidePoints, const CRect& rectClip)
{
	
	if(polygon.size()<3)	//at least three points
		return;
	
	vector vInputPos;
	vector vOutputPos;
	vInputPos.resize(polygon.size());
                                    
                                    
                                        
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