[rdf] fix windows warning about precision lost in RDisplay::fWidth

Here size_t variable converted into unsigned short - windows complains.
Original message:

RDFDisplay.cxx(96): warning C4267: '=': conversion from 'size_t' to
'_Ty', possible loss of data 

RDFDisplay.cxx(117): warning C4267: '=': conversion from 'size_t' to
'_Ty', possible loss of data