R_FntNr()
 Retrieve the number of the current font
------------------------------------------------------------------------------

 Syntax
        cFontNr := R_FntNr ()

 Arguments
        None

 Returns
        nFontNr         Number of the current screen font.
                        1) For the Default ROM Font :
                                cFontNr = -1
                        2) For an INTERNAL font :
                                cFontNr = Number of the internal font
                                (Range: 1 .. R_MaxFnt())
                        3) For an EXTERNAL font, loaded from FILE :
                                cFontNr = 0
                        4) For an EXTERNAL font, loaded from STRING :
                                cFontNr = 0

 Description
        Retrieves the number of the current screen font.

 Example
        *-- Get number of current font
        LOCAL   nFontNr := R_FntNr ()

        DO CASE
        CASE nFontNr = -1
                Alert ( 'Current font is : DEFAULT ROM font' )
        CASE nFontNr = 0
                Alert ( 'Current font is : EXTERNAL' )
        OTHERWISE
                Alert ( 'Current font is : INTERNAL # ' + Str ( nFontNr, 1 ) )
        ENDCASE