Compute a normal matrix from the upper left 3x3 submatrix of <code>this</code>
and store it into the upper left 3x3 submatrix of <code>this</code>.
All other values of <code>this</code> will be set to {@link #identity() identity}.
<p>
The normal matrix of <code>m</code> is the transpose of the inverse of <code>m</code>.
<p>
Please note that, if <code>this</code> is an orthogonal matrix or a matrix whose columns are orthogonal vectors,
then this method <i>need not</i> be invoked, since in that case <code>this</code> itself is its normal matrix.
In that case, use {@link #set3x3(Matrix4d)} to set a given Matrix4f to only the upper left 3x3 submatrix
of this matrix.
Compute a normal matrix from the upper left 3x3 submatrix of <code>this</code> and store it into the upper left 3x3 submatrix of <code>this</code>. All other values of <code>this</code> will be set to {@link #identity() identity}. <p> The normal matrix of <code>m</code> is the transpose of the inverse of <code>m</code>. <p> Please note that, if <code>this</code> is an orthogonal matrix or a matrix whose columns are orthogonal vectors, then this method <i>need not</i> be invoked, since in that case <code>this</code> itself is its normal matrix. In that case, use {@link #set3x3(Matrix4d)} to set a given Matrix4f to only the upper left 3x3 submatrix of this matrix.
@see #set3x3(Matrix4d)
@return this