core: Ignore newer PlaceObject tag in same frame with duplicate depth

If we have two PlaceObject tags in the same frame with the same depth,
only the first one actually places an object. The second one is ignored
(Flash Player logs a warning).

core/src/display_object/movie_clip.rs

@@ -4439,7 +4439,12 @@ impl QueuedTagList {
     fn queue_add(&mut self, add_tag: QueuedTag) {
         let new = match self {
             QueuedTagList::None => QueuedTagList::Add(add_tag),
-            QueuedTagList::Add(_) => QueuedTagList::Add(add_tag),
+            QueuedTagList::Add(existing) => {
+                // Flash player traces "Warning: Failed to place object at depth 1.",
+                // so let's log a warning too.
+                tracing::warn!("Ignoring queued tag {add_tag:?} at same depth as {existing:?}");
+                QueuedTagList::Add(*existing)
+            }
             QueuedTagList::Remove(r) => QueuedTagList::RemoveThenAdd(*r, add_tag),
             QueuedTagList::RemoveThenAdd(r, _) => QueuedTagList::RemoveThenAdd(*r, add_tag),
         };

tests/tests/swfs/avm2/place_object_same_depth_frame/FirstChild.as

@@ -0,0 +1,14 @@
+package  {
+	
+	import flash.display.MovieClip;
+	
+	
+	public class FirstChild extends MovieClip {
+		
+		
+		public function FirstChild() {
+			trace("Constructed FirstChild");
+		}
+	}
+	
+}

tests/tests/swfs/avm2/place_object_same_depth_frame/SecondChild.as

@@ -0,0 +1,14 @@
+package  {
+	
+	import flash.display.MovieClip;
+	
+	
+	public class SecondChild extends MovieClip {
+		
+		
+		public function SecondChild() {
+			trace("Constructed SecondChild")
+		}
+	}
+	
+}

tests/tests/swfs/avm2/place_object_same_depth_frame/output.txt

@@ -0,0 +1 @@
+Constructed FirstChild

tests/tests/swfs/avm2/place_object_same_depth_frame/test.toml

@@ -0,0 +1,3 @@
+# NOTE - test.swf is manually edited to have two PlaceObject tags
+# in the same frame with the same depth
+num_frames = 1