Predictable output for vtkTensorGlyph on non-symmetric tensors

Previous version implicitly assumed that the tensor was symmetric when using eigenvalue decomposition. General tensors have non-real eigenvalues and eigenvectors. It did not crash on general input because vtkMath::Jacobi expects real symmetric matrix, and only use the upper right triangular part of the matrix. So computations were as if the lower triangular part of the tensor was symmetric of its the upper triangular part. It make more sense to apply the eigenvalue decomposition to the symmetrical part of the tensor.

Closes #17132 (closed)