Fix T49180: Cycles MIS Map for Animated Environment Texture Movie Doesn't Update on Frame Change

Not really ideal fix at all, but we are at RC today, so better to play really safe.

intern/cycles/render/shader.cpp

@@ -221,6 +221,16 @@ void Shader::tag_update(Scene *scene)
 	if(use_mis && has_surface_emission)
 		scene->light_manager->need_update = true;
 
+	/* Special handle of background MIS light for now: for some reason it
+	 * has use_mis set to false. We are quite close to release now, so
+	 * better to be safe.
+	 */
+	if(this == scene->default_background &&
+	   scene->light_manager->has_background_light(scene))
+	{
+		scene->light_manager->need_update = true;
+	}
+
 	/* quick detection of which kind of shaders we have to avoid loading
 	 * e.g. surface attributes when there is only a volume shader. this could
 	 * be more fine grained but it's better than nothing */