TEveRGBAPalette:
Add support for displaying palette values in UI elements (GUI, overlay) as
real values, mapped with a linear function. This is enabled by calling:
TEveRGBAPalette::SetUIDoubleRep(Bool_t b, Double_t f=1, Double_t c=0);
and the mapping between integer 'i' and real 'r' values is:
r = f*i + c;