HOLLAND — Work has begun on a mixed-use building project that will change the landscape of downtown Holland.The two-story building at 159 S. River Ave., once the Holland Rescue Mission’s youth center, is in the process of being demolished this week.In its place, a five-story project called Towers on River is expected to transform the entire block between Sixth and Seventh streets.The builders plan to build for retail tenants on the first floor, medical office space on the second