Add test coverage of storyboard animation start time

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

@@ -97,6 +97,25 @@ namespace osu.Game.Tests.Beatmaps.Formats
             }
         }
 
+        [Test]
+        public void TestCorrectAnimationStartTime()
+        {
+            var decoder = new LegacyStoryboardDecoder();
+
+            using (var resStream = TestResources.OpenResource("animation-starts-before-alpha.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);
+                // This property should be used in DrawableStoryboardAnimation as a starting point for animation playback.
+                Assert.AreEqual(1000, (background.Elements[0] as StoryboardAnimation)?.EarliestTransformTime);
+            }
+        }
+
         [Test]
         public void TestOutOfOrderStartTimes()
         {

osu.Game.Tests/Resources/animation-starts-before-alpha.osb

@@ -0,0 +1,5 @@
+[Events]
+//Storyboard Layer 0 (Background)
+Animation,Background,Centre,"img.jpg",320,240,2,150,LoopForever
+ S,0,1000,1500,0.08 // animation should start playing from this point in time..
+ F,0,2000,,0,1      // .. not this point in time