update ospray.h to define NULL appropriately based on C++ standard used
update ospray.h to define NULL appropriately based on C++ standard used

favreau/OSPRay,Twinklebear/OSPRay,favreau/OSPRay,ospray/OSPRay,wilsonCernWq/ospray,ospray/OSPRay,ospray/OSPRay,MengjiaoH/ospray,favreau/OSPRay,MengjiaoH/ospray,Twinklebear/OSPRay,MengjiaoH/ospray,Twinklebear/OSPRay,wilsonCernWq/ospray,MengjiaoH/ospray,wilsonCernWq/ospray,ospray/OSPRay,MengjiaoH/ospray,Twinklebear/OSPRay,favreau/OSPRay