Add test coverage of expected behaviour for playback of loops with no explicit end time

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

@@ -95,6 +95,25 @@ public void TestDecodeStoryboardEvents()
             }
         }
 
+        [Test]
+        public void TestLoopWithoutExplicitFadeOut()
+        {
+            var decoder = new LegacyStoryboardDecoder();
+
+            using (var resStream = TestResources.OpenResource("animation-loop-no-explicit-end-time.osb"))
+            using (var stream = new LineBufferedReader(resStream))
+            {
+                var storyboard = decoder.Decode(stream);
+
+                StoryboardLayer background = storyboard.Layers.Single(l => l.Depth == 3);
+                Assert.AreEqual(1, background.Elements.Count);
+
+                Assert.AreEqual(2000, background.Elements[0].StartTime);
+                Assert.AreEqual(2000, (background.Elements[0] as StoryboardAnimation)?.EarliestTransformTime);
+                Assert.AreEqual(12000, (background.Elements[0] as StoryboardAnimation)?.GetEndTime());
+            }
+        }
+
         [Test]
         public void TestCorrectAnimationStartTime()
         {

osu.Game.Tests/Resources/animation-loop-no-explicit-end-time.osb

@@ -0,0 +1,6 @@
+[Events]
+//Storyboard Layer 0 (Background)
+Animation,Background,Centre,"img.jpg",320,240,2,150,LoopForever
+ F,0,2000,,0,1
+ L,2000,10
+  F,18,0,1000,1,0