Adjust song select beatmap background transition to better support transparent backgrounds

The new background now briefly fades in. The reason we didn't do this to
date is that there could be a perceived decrease in brightness as the old
and new background transition through opacity.

But a quick fade in, it doesn't seem to cause any visual artifacting.

I've also added a scale effect because it felt quite nice. Willing to
pull that if anyone has an issue with it, but it's a step in the
direction of "adding more motion to song select", which is still an area
I see lacking greatly – even compared to stable.

osu.Game/Screens/Backgrounds/BackgroundScreenBeatmap.cs

@@ -108,12 +108,14 @@ namespace osu.Game.Screens.Backgrounds
             if (Background != null)
             {
                 newDepth = Background.Depth + 1;
-                Background.FinishTransforms();
                 Background.FadeOut(250);
                 Background.Expire();
             }
 
             b.Depth = newDepth;
+            b.Anchor = b.Origin = Anchor.Centre;
+            b.FadeInFromZero(500, Easing.OutQuint);
+            b.ScaleTo(1.02f).ScaleTo(1, 3500, Easing.OutQuint);
             dimmable.Background = Background = b;
         }