Implement execute once annotation on properties.

Assets/Cryville/Crtr/SkinContainer.cs

@@ -79,6 +79,9 @@ namespace Cryville.Crtr {
 			for (int i = 0; i < stack.Properties.Count; i++) {
 				DynamicProperty p = stack.Properties[i];
 				p.Key.ExecuteDynamic(_group, p.Context, p.Value, dl);
+				if (p.Key.annotations.Contains("once")) {
+					stack.Properties.RemoveAt(i--);
+				}
 			}
 			for (int i = 0; i < stack.Elements.Count; i++) {
 				DynamicElement e = stack.Elements[i];