feat(sdk): BufferedGroups can be used as masks

Author: CGjupoulton

engine/modules/entities/src/main/java/com/codingame/gameengine/module/entities/BufferedGroup.java

@@ -7,7 +7,7 @@
  * This reduces artifacts caused by rounding errors at the cost of lower performance.
  * </p>
  */
-public class BufferedGroup extends ContainerBasedEntity<BufferedGroup> {
+public class BufferedGroup extends ContainerBasedEntity<BufferedGroup> implements Mask {
     @Override
     Entity.Type getType() {
         return Entity.Type.BUFFERED_GROUP;