feat: update ordering

Memola/Canvas/Elements/Core/Element.swift

@@ -40,4 +40,17 @@ enum Element: Equatable, Comparable {
             photo.createdAt
         }
     }
+
+    static func < (lhs: Element, rhs: Element) -> Bool {
+        switch (lhs, rhs) {
+        case let (.stroke(leftStroke), .stroke(rightStroke)):
+            leftStroke < rightStroke
+        case let (.photo(leftPhoto), .photo(rightPhoto)):
+            leftPhoto < rightPhoto
+        case let (.photo(photo), .stroke(stroke)):
+            photo.createdAt < stroke.value.createdAt
+        case let (.stroke(stroke), .photo(photo)):
+            stroke.value.createdAt < photo.createdAt
+        }
+    }
 }

Memola/Canvas/RTree/RTree.swift

@@ -38,7 +38,11 @@ class RTree<T> where T: Equatable & Comparable {
                     .sorted(by: <)
                 result = _merge(result, children)
             } else {
-                queue.append(contentsOf: node.children)
+                let nodes = node.children.sorted {
+                    guard let first = $0.value, let second = $1.value else { return false }
+                    return first < second
+                }
+                queue.append(contentsOf: nodes)
             }
         }
         return result