Fix #32205: Holdout shader + transparent background stays black with Alpha = 1

It was read of initialized memory around holdout_weight in cases when
holdout material is used. Seems that it should be assigned to result
of shader_holdout_eval here.

If Brecht could double check this it'll be great.

This could potentially fix #32224: Holdout Error with CUDA Cycles Render

intern/cycles/kernel/kernel_path.h

@@ -288,7 +288,7 @@ __device float4 kernel_path_progressive(KernelGlobals *kg, RNG *rng, int sample,
 				if(sd.flag & SD_HOLDOUT_MASK)
 					holdout_weight = make_float3(1.0f, 1.0f, 1.0f);
 				else
-					shader_holdout_eval(kg, &sd);
+					holdout_weight = shader_holdout_eval(kg, &sd);
 
 				/* any throughput is ok, should all be identical here */
 				L_transparent += average(holdout_weight*throughput);
@@ -655,7 +655,7 @@ __device float4 kernel_path_non_progressive(KernelGlobals *kg, RNG *rng, int sam
 				if(sd.flag & SD_HOLDOUT_MASK)
 					holdout_weight = make_float3(1.0f, 1.0f, 1.0f);
 				else
-					shader_holdout_eval(kg, &sd);
+					holdout_weight = shader_holdout_eval(kg, &sd);
 
 				/* any throughput is ok, should all be identical here */
 				L_transparent += average(holdout_weight*throughput);