Implement null transmission evaluation for Mask BSDF

Author: Delio Vicini

src/bsdfs/mask.cpp

@@ -153,6 +153,12 @@ class MaskBSDF final : public BSDF<Float, Spectrum> {
 
         return result;
     }
+        
+    Spectrum eval_null_transmission(const SurfaceInteraction3f &si,
+                                    Mask active) const override {
+        Float opacity = eval_opacity(si, active);
+        return 1 - opacity * (1 - m_nested_bsdf->eval_null_transmission(si, active));
+    }
 
     MTS_INLINE Float eval_opacity(const SurfaceInteraction3f &si, Mask active) const {
         return clamp(m_opacity->eval_1(si, active), 0.f, 1.f);