Fix beatmap carousel not preloading panels when off-screen

[peppy]

Something I noticed in passing that felt like it deserved 45 minutes of attention.

Before:

CleanShot.2024-01-04.at.10.16.09.mp4

After:

CleanShot.2024-01-04.at.10.22.56.mp4

The preload numbers are still on the conservative side. We can probably adjust these based on the system the game is running on in the future. Or across the board. But this should improve the overall responsiveness of carousel loading, something that's been bugging me for quite a while now (and not just me)

For testing purposes (to see when loads are triggered):

diff --git a/osu.Game/Screens/Select/Carousel/DrawableCarouselBeatmapSet.cs b/osu.Game/Screens/Select/Carousel/DrawableCarouselBeatmapSet.cs
index 119818242e..328678cd2b 100644
--- a/osu.Game/Screens/Select/Carousel/DrawableCarouselBeatmapSet.cs
+++ b/osu.Game/Screens/Select/Carousel/DrawableCarouselBeatmapSet.cs
@@ -13,6 +13,7 @@
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Graphics.Cursor;
 using osu.Framework.Graphics.Primitives;
+using osu.Framework.Graphics.Shapes;
 using osu.Framework.Graphics.UserInterface;
 using osu.Framework.Utils;
 using osu.Game.Beatmaps;
@@ -20,6 +21,7 @@
 using osu.Game.Database;
 using osu.Game.Graphics.UserInterface;
 using osu.Game.Overlays;
+using osuTK.Graphics;
 
 namespace osu.Game.Screens.Select.Carousel
 {
@@ -203,6 +205,14 @@ private void loadContentIfRequired()
             {
                 loadCancellation = new CancellationTokenSource();
 
+                var drawable = new Box
+                {
+                    Colour = Color4.Red,
+                    RelativeSizeAxes = Axes.Both,
+                };
+
+                Header.Add(drawable);
+
                 LoadComponentsAsync(new CompositeDrawable[]
                 {
                     new SetPanelBackground(manager.GetWorkingBeatmap(beatmapSet.Beatmaps.MinBy(b => b.OnlineID)))
@@ -218,6 +228,7 @@ private void loadContentIfRequired()
                 {
                     Header.AddRange(drawables);
                     drawables.ForEach(d => d.FadeInFromZero(150));
+                    drawable.Colour = Color4.White;
                 }, loadCancellation.Token);
             }
         }

[bdach]

don't get it

[bdach]

i worry about the performance implications of this but we won't see how it goes without trying it i guess