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.