Restructure event/container system.

Assets/Cryville/Crtr/StampedEvent.cs

@@ -39,8 +39,16 @@ namespace Cryville.Crtr {
 			public override int Priority { get { return m_priority; } }
 			public void SetPriority(int value) { m_priority = value; }
 		}
+
+		public class ClipBehind : StampedEvent {
 			public override int Priority {
-				get { return 0; }
+				get { return Origin.Priority - 0x8000; }
+			}
+		}
+
+		public class ClipAhead : StampedEvent {
+			public override int Priority {
+				get { return 0x7fff - Origin.Priority; }
 			}
 		}