Sorts the scene entries by path before serializing. (#15047)

# Objective

Fixes: #14515

## Solution

Sorts the iterator with itertools' sorted_by function. This is required
given that 'self.entries' is an immutable &[Box<dyn PartialReflect]
which also doesn't implement Clone or Copy.

## Testing

The modifications passed the unit testing only after they were edited to
ensure that the items were in alphabetical order.
I haven't checked for performance implications.

crates/bevy_scene/src/serde.rs

@@ -154,6 +154,8 @@ impl<'a> Serialize for EntitySerializer<'a> {
 /// Used to serialize scene resources in [`SceneSerializer`] and entity components in [`EntitySerializer`].
 /// Note that having several entries of the same type in `entries` will lead to an error when using the RON format and
 /// deserializing through [`SceneMapDeserializer`].
+///
+/// Note: The entries are sorted by type path before they're serialized.
 pub struct SceneMapSerializer<'a> {
  /// List of boxed values of unique type to serialize.
  pub entries: &'a [Box<dyn PartialReflect>],
@@ -167,10 +169,25 @@ impl<'a> Serialize for SceneMapSerializer<'a> {
  S: Serializer,
  {
  let mut state = serializer.serialize_map(Some(self.entries.len()))?;
- for reflect in self.entries {
+ let sorted_entries = {
+ let mut entries = self
+ .entries
+ .iter()
+ .map(|entry| {
+ (
+ entry.get_represented_type_info().unwrap().type_path(),
+ entry.as_partial_reflect(),
+ )
+ })
+ .collect::<Vec<_>>();
+ entries.sort_by_key(|(type_path, _partial_reflect)| *type_path);
+ entries
+ };
+
+ for (type_path, partial_reflect) in sorted_entries {
  state.serialize_entry(
- reflect.get_represented_type_info().unwrap().type_path(),
- &TypedReflectSerializer::new(reflect.as_partial_reflect(), self.registry),
+ type_path,
+ &TypedReflectSerializer::new(partial_reflect, self.registry),
  )?;
  }
  state.end()
@@ -598,15 +615,15 @@ mod tests {
  ),
  4294967297: (
  components: {
- "bevy_scene::serde::tests::Foo": (123),
  "bevy_scene::serde::tests::Bar": (345),
+ "bevy_scene::serde::tests::Foo": (123),
  },
  ),
  4294967298: (
  components: {
- "bevy_scene::serde::tests::Foo": (123),
  "bevy_scene::serde::tests::Bar": (345),
  "bevy_scene::serde::tests::Baz": (789),
+ "bevy_scene::serde::tests::Foo": (123),
  },
  ),
  },