Fix main loop, add example and cleanup

include/utils.h

@@ -158,11 +158,6 @@ struct Rect {
 
 template <typename T>
 struct Pool {
-  Pool() {
-    items.reserve(20);
-    free_indices.reserve(20);
-  }
-
   T &operator[](int index) {
     return items[index];
   }
@@ -175,10 +170,10 @@ struct Pool {
     if (!free_indices.empty()) {
       int index = free_indices.back();
       free_indices.pop_back();
-      items[index] = item;
+      items[index] = std::move(item);
       return index;
     } else {
-      items.push_back(item);
+      items.push_back(std::move(item));
       return items.size() - 1;
     }
   }