add method
Add a to this matrix
Implementation
Mat33 add(Mat33 a, [Mat33? b ]) {
if( b != null ) return addMatrixs( a, b );
var e = elements, te = a.elements;
e[0] += te[0]; e[1] += te[1]; e[2] += te[2];
e[3] += te[3]; e[4] += te[4]; e[5] += te[5];
e[6] += te[6]; e[7] += te[7]; e[8] += te[8];
return this;
}