This patch implements an "indexed" color mode for `vtkLookupTable`.
When `IndexedLookup` is true, the scalar range and log/linear settings are disabled;
`MapScalars` and `MapValue` require an exact match to an annotated value
in order to return a valid color.
All other values are mapped to `NanColor`.

Change-Id: I9805ab6b4398db479c0ae6b0d239886850c2e187