void
 clear()
 {
     m_data.clear();
     m_dims = FgVect2UI(0);
 }
 // WARNING: This does not adjust any existing image data, just allocated dimensions and memory:
 void
 resize(uint wid,uint hgt)
 {
     m_dims = FgVect2UI(wid,hgt);
     m_data.resize(wid*hgt);
 }
 FgVect2UI       dims() const {return FgVect2UI(ncols,nrows); }  // NB Order