Make AsQuaternion a const member function (cherry-picked from aedra svn revision 1).

include/nif_math.h

@@ -621,7 +621,7 @@ struct Matrix33 {
 	/*! Returns a quaternion representation of the rotation stored in this matrix. 
 	 * \return A quaternion with an equivalent rotation to the one stored in this matrix.
 	 */
-	NIFLIB_API Quaternion AsQuaternion();
+	NIFLIB_API Quaternion AsQuaternion() const;
 
 	/*! Calculates the determinant of this matrix.
 	 * \return The determinant of this matrix.

src/nif_math.cpp

@@ -298,14 +298,14 @@ Matrix33::Matrix33() {
 	*this = Matrix33::IDENTITY;
 }
 
-Quaternion Matrix33::AsQuaternion() {
+Quaternion Matrix33::AsQuaternion() const {
 	Quaternion quat;
 	float tr, s, q[4];
 	int i, j, k;
 
 	int nxt[3] = {1, 2, 0};
 
-	Matrix33 & m = *this;
+	const Matrix33 & m = *this;
 
 	// compute the trace of the matrix
 	tr = m[0][0] + m[1][1] + m[2][2];