Allow GetData from const vtkMatrix

Add overloads of GetData to vtkMatrix3x3, vtkMatrix4x4 to allow
accessing the (immutable) underlying data of a const-qualified instance.

Common/Math/vtkMatrix3x3.h

@@ -154,6 +154,11 @@ public:
    */
   double * GetData() { return *this->Element; }
 
+  /**
+   * Return a pointer to the first element of the matrix (double[9]).
+   */
+  const double * GetData() const { return *this->Element; }
+
 protected:
   vtkMatrix3x3();
   ~vtkMatrix3x3() override;

Common/Math/vtkMatrix4x4.h

@@ -170,6 +170,11 @@ public:
    */
   double *GetData() { return *this->Element; }
 
+  /**
+   * Returns the raw double array holding the matrix.
+   */
+  const double *GetData() const { return *this->Element; }
+
 protected:
   vtkMatrix4x4() { vtkMatrix4x4::Identity(*this->Element); };
   ~vtkMatrix4x4() override {}