BUG 15903: Fix race condition in setting special colors in table

In vtkLookupTableMapData(), the special colors were set in the lookup table prior to doing the mapping. This can lead to a race condition if multiple threads call this function at the same time. Normally, this shouldn't be a problem, because if the vtkLookupTable used to fill in the table values is the same, each thread will write the same values to each entry in the table. Some verification tools, however, will note that more than one thread is writing to the same memory location at a time.

Avoid this by setting the special colors when the lookup table is built or when custom table values are set.