BUG #13776. Fixed mtime computation.

vtkDiscretizableColorTransferFunction was not updating its MTime when the
optional ScalarOpacityFunction itself was modified. That was resulting in the
LUT not being regenerated on changes to ScalarOpacityFunction, manifested in
BUG# 13776. This patch address that issue.

Change-Id: If1b63710a6739870f55b089aabc38b7c93c0e493
Merge topic '13776_fix_lut_updates' into master

fa104875 BUG #13776. Fixed mtime computation.