fix deadlock problem --> slave never called DFB::endFrame()
fix deadlock problem --> slave never called DFB::endFrame()

[ci skip]

wilsonCernWq/ospray,MengjiaoH/ospray,ospray/OSPRay,MengjiaoH/ospray,ospray/OSPRay,wilsonCernWq/ospray,MengjiaoH/ospray,MengjiaoH/ospray,wilsonCernWq/ospray,ospray/OSPRay,MengjiaoH/ospray,ospray/OSPRay