Fix T51805: Overlapping volumes renders incorrect on AMD GPU

We need to make sure we can store all volume closures for all objects in volume
stack. This is a bit tricky to detect what would be the "nestness" level of
volumes so for now use maximum possible stack depth. Might cause some slowdown,
but better to give reliable render output than to fail quickly.

Should be safe for 2.79 after extra eyes.

intern/cycles/render/graph.cpp

@@ -1018,6 +1018,9 @@ int ShaderGraph::get_num_closures()
 		else if(CLOSURE_IS_PRINCIPLED(closure_type)) {
 			num_closures += 8;
 		}
+		else if(CLOSURE_IS_VOLUME(closure_type)) {
+			num_closures += VOLUME_STACK_SIZE;
+		}
 		else {
 			++num_closures;
 		}