[SharpMap] 屏幕坐标和Map坐标转换

if (m_commandType == eCommandType.pan && e.Button == MouseButtons.Left)
{
m_dragOffset.X = -(m_mousedownPoint.X - e.X);
m_dragOffset.Y = -(m_mousedownPoint.Y - e.Y);
m_lastCenterPoint = CenterPointUnit();
DoInvalidate(true);
}