Handle automapper realm cyclic references via AfterMaps

This may not be the cleanest solution, but there don't seem to be any
way towards this either.

- `UseDestinationValue` has been inherited by default as noted in
https://docs.automapper.org/en/stable/10.0-Upgrade-Guide.html#usedestinationvalue-is-now-inherited-by-default, and its behaviour in this case would be using the nested **managed** realm object for the destination member rather than creating an unmanaged version.

- `MaxDepth` already sets `PreserveReferences` so there's no point of using it.

- `MaxDepth` should probably not be set for all maps, only for those with
cyclic references, to avoid the expensive overhead of `PreserveReferences`, as mentioned in https://docs.automapper.org/en/stable/5.0-Upgrade-Guide.html#circular-references.

That aside, `MaxDepth` should actually only be set to `1` for
`BeatmapSetInfo` mapping, because we don't want AutoMapper to create a
nested instance of `BeatmapSetInfo` in each mapped/detached beatmap, but
for some reason, doing that will cause automapper to not map any beatmap
inside the set and leave it with 0 beatmaps.

While on the other hand, using `MaxDepth(2)` for `BeatmapSetInfo` works,
but creates an unused instance of a `BeatmapSetInfo` inside each mapped
beatmap, which may not be ideal.

For `BeatmapInfo`, it has to be `MaxDepth(2)`, in which the first
`BeatmapInfo` depth would be itself (when detaching a beatmap), and the
second would be nested beatmaps inside the mapped/detached
`BeatmapSetInfo` within the beatmap. (note that when detaching a beatmap
set, the unused instance of `BeatmapSetInfo` within each beatmap of that
beatmap set doesn't also have a list of unused beatmaps as one might expect from the depth specification, it surprisingly has 0 beatmaps)

This causes it to create an unused instance of `BeatmapInfo` in the beatmap set resembling the root mapped/detached beatmap, but that one might be inevitable.

osu.Game/Database/RealmObjectExtensions.cs

@@ -34,15 +34,23 @@ namespace osu.Game.Database
             c.CreateMap<RealmUser, RealmUser>();
             c.CreateMap<RealmFile, RealmFile>();
             c.CreateMap<RealmNamedFileUsage, RealmNamedFileUsage>();
-            c.CreateMap<BeatmapInfo, BeatmapInfo>();
-            c.CreateMap<BeatmapSetInfo, BeatmapSetInfo>();
-            c.AddGlobalIgnore(nameof(RealmObjectBase.ObjectSchema));
-
-            c.ForAllMaps((a, b) =>
+            c.CreateMap<BeatmapInfo, BeatmapInfo>().MaxDepth(2).AfterMap((s, d) =>
             {
-                b.PreserveReferences();
-                b.MaxDepth(2);
+                for (int i = 0; i < d.BeatmapSet?.Beatmaps.Count; i++)
+                {
+                    if (d.BeatmapSet.Beatmaps[i].Equals(d))
+                    {
+                        d.BeatmapSet.Beatmaps[i] = d;
+                        break;
+                    }
+                }
             });
+            c.CreateMap<BeatmapSetInfo, BeatmapSetInfo>().MaxDepth(2).AfterMap((s, d) =>
+            {
+                foreach (var beatmap in d.Beatmaps)
+                    beatmap.BeatmapSet = d;
+            });
+            c.AddGlobalIgnore(nameof(RealmObjectBase.ObjectSchema));
         }).CreateMapper();
 
         private static readonly IMapper read_mapper = new MapperConfiguration(c =>
@@ -60,15 +68,23 @@ namespace osu.Game.Database
             c.CreateMap<RealmUser, RealmUser>();
             c.CreateMap<RealmFile, RealmFile>();
             c.CreateMap<RealmNamedFileUsage, RealmNamedFileUsage>();
-            c.CreateMap<BeatmapInfo, BeatmapInfo>();
-            c.CreateMap<BeatmapSetInfo, BeatmapSetInfo>();
-            c.AddGlobalIgnore(nameof(RealmObjectBase.ObjectSchema));
-
-            c.ForAllMaps((a, b) =>
+            c.CreateMap<BeatmapInfo, BeatmapInfo>().MaxDepth(2).AfterMap((s, d) =>
             {
-                b.PreserveReferences();
-                b.MaxDepth(2);
+                for (int i = 0; i < d.BeatmapSet?.Beatmaps.Count; i++)
+                {
+                    if (d.BeatmapSet.Beatmaps[i].Equals(d))
+                    {
+                        d.BeatmapSet.Beatmaps[i] = d;
+                        break;
+                    }
+                }
             });
+            c.CreateMap<BeatmapSetInfo, BeatmapSetInfo>().MaxDepth(2).AfterMap((s, d) =>
+            {
+                foreach (var beatmap in d.Beatmaps)
+                    beatmap.BeatmapSet = d;
+            });
+            c.AddGlobalIgnore(nameof(RealmObjectBase.ObjectSchema));
         }).CreateMapper();
 
         /// <summary>