copy method
Implementation
Matrix3 copy(m) {
var te = elements;
var me = m.elements;
te[0] = me[0];
te[1] = me[1];
te[2] = me[2];
te[3] = me[3];
te[4] = me[4];
te[5] = me[5];
te[6] = me[6];
te[7] = me[7];
te[8] = me[8];
return this;
}