In the case where one of R6 to R8 is a hydrogen atom or none of these is a hydrogen atom (e.g., in the case where R6 is a hydrogen atom and R7 and R8 each is a group represented by --COOR13 or where R6 to R8 each is a group represented by --COOR13), then the alkenyl group represented by R13 or R14 in each of the remaining two or all of R6 to R8 is preferably a linear or branched alkenyl group havi