COMP: resolve clamp() float/double ambiguity (SPDP compilation)

- pass by value instead of reference, add functional casts in some
  places. Can still rely on integer promotions though.

  OK:   clamp(value, 2, 20)   ==> (float, int, int)
  OK:   clamp(value, scalar(2), scalar(20))  ==> (float, float, float)
  NOK:  clamp(value, 2.0, 20) ==> (float, double, int)