Handle background offset when encoding/decoding beatmaps (#37841)

- Part of https://github.com/ppy/osu/issues/14238 (doesn't close,
because the property doesn't do anything yet).
- Supersedes / closes https://github.com/ppy/osu/pull/37467.

osu.Game.Tests/Beatmaps/Formats/LegacyStoryboardEncoderTest.cs

@@ -30,6 +30,23 @@ namespace osu.Game.Tests.Beatmaps.Formats
             Assert.That(decodedAfterEncode.Beatmap.BeatmapInfo.Metadata.BackgroundFile, Is.EqualTo("bg.jpg"));
         }
 
+        [Test]
+        public void TestBackgroundOffset()
+        {
+            var initial = createComponents();
+            initial.Beatmap.BeatmapInfo.Metadata.BackgroundFile = "bg_offset.jpg";
+            initial.Storyboard.BackgroundOffset = new Vector2(0, 45);
+
+            var encoded = encode(initial);
+            var decodedAfterEncode = decode(encoded);
+
+            Assert.Multiple(() =>
+            {
+                Assert.That(decodedAfterEncode.Beatmap.BeatmapInfo.Metadata.BackgroundFile, Is.EqualTo("bg_offset.jpg"));
+                Assert.That(decodedAfterEncode.Storyboard.BackgroundOffset, Is.EqualTo(new Vector2(0, 45)));
+            });
+        }
+
         [Test]
         public void TestVideos()
         {

osu.Game/Beatmaps/Formats/LegacyStoryboardDecoder.cs

@@ -120,6 +120,20 @@ namespace osu.Game.Beatmaps.Formats
 
                 switch (type)
                 {
+                    case LegacyEventType.Background:
+                    {
+                        // the actual filename is handled in `LegacyBeatmapDecoder`.
+                        // this only handles the background offset, because it does not logically belong in `Beatmap` or related classes.
+                        if (split.Length > 4)
+                        {
+                            float x = Parsing.ParseFloat(split[3]);
+                            float y = Parsing.ParseFloat(split[4]);
+                            storyboard.BackgroundOffset = new Vector2(x, y);
+                        }
+
+                        break;
+                    }
+
                     case LegacyEventType.Video:
                     {
                         int offset = Parsing.ParseInt(split[1]);

osu.Game/Beatmaps/Formats/LegacyStoryboardEncoder.cs

@@ -54,10 +54,9 @@ namespace osu.Game.Beatmaps.Formats
             if (target == StoryboardElementSource.Beatmap)
             {
                 // https://github.com/peppy/osu-stable-reference/blob/c34a74fb61c17c5667486a12548485d1f03baa2e/osu!/GameplayElements/HitObjectManager_LoadSave.cs#L1499
-                // TODO: handle nonzero background offset (https://github.com/ppy/osu/issues/14238)
                 writer.WriteLine(string.Format(CultureInfo.InvariantCulture,
                     @"{0},{1},""{2}"",{3},{4}",
-                    (int)LegacyEventType.Background, 0, storyboard.BeatmapInfo.Metadata.BackgroundFile, 0, 0));
+                    (int)LegacyEventType.Background, 0, storyboard.BeatmapInfo.Metadata.BackgroundFile, storyboard.BackgroundOffset.X, storyboard.BackgroundOffset.Y));
             }
 
             // https://github.com/peppy/osu-stable-reference/blob/c34a74fb61c17c5667486a12548485d1f03baa2e/osu!/GameplayElements/HitObjectManager_LoadSave.cs#L1496

osu.Game/Storyboards/Storyboard.cs

@@ -9,6 +9,7 @@ using osu.Game.Beatmaps;
 using osu.Game.Rulesets.Mods;
 using osu.Game.Storyboards.Drawables;
 using osu.Game.Utils;
+using osuTK;
 
 namespace osu.Game.Storyboards
 {
@@ -98,6 +99,12 @@ namespace osu.Game.Storyboards
             }
         }
 
+        /// <summary>
+        /// Offset to be applied to the beatmap background.
+        /// TODO: Unused yet. See https://github.com/ppy/osu/issues/14238.
+        /// </summary>
+        public Vector2 BackgroundOffset { get; set; } = Vector2.Zero;
+
         public virtual DrawableStoryboard CreateDrawable(IReadOnlyList<Mod>? mods = null) =>
             new DrawableStoryboard(this, mods);