Add timestamp check for vtkLookupTable BuildSpecialColors
This will re-build the BelowRange, AboveRange, and Nan color entries in the lookup table if they are out of date. Without this change, if a user calls SetBelowRangeColor() etc. after the table has been set or built, then the table will remain unchanged. I have modified the test so that it will fail without this patch.
Fixes #16966 (closed)