when ray exceeds `max_bounce`, we do not allocate any closure at
intersection. However, Ray Portal BSDF still added `SD_BSDF` flag,
resulting in undefined behavior in
`integrate_surface_bsdf_bssrdf_bounce()`.
This part of code was similar to Transparent BSDF, however, Transparent
closure was still allocated in this case.
To fix the undefined behavior, add `SD_BSDF` flag only when the Ray
Portal closure was allocated.
Weizhen Huang
68de483bf3