Fix TextureLoaderStore not always appending extensions to lookups

[frenzibyte]

• Closes Mods are not displayed correctly on tournament map pool screen.  osu#18989

Tournament texture storage uses a StorageBackedResourceStore, which is not of type ResourceStore, therefore TextureLoaderStore can't append the image extensions and fail lookups:

osu-framework/osu.Framework/Graphics/Textures/TextureLoaderStore.cs

Lines 24 to 29 in e918d33

	public TextureLoaderStore(IResourceStore<byte[]> store)
	{
	this.store = store;
	(store as ResourceStore<byte[]>)?.AddExtension(@"png");
	(store as ResourceStore<byte[]>)?.AddExtension(@"jpg");
	}

This is a regression from #5240, since prior to that PR, TextureStore appends extensions as well (this is also the reason why previously texture store sometimes looks up textures with double extensions, i.e. texture.jpg.png).

Instead, I've changed TextureLoaderStore to explicitly construct a resource store and add the given store along with the right extensions there, rather than safely cast and silently fail.

Note that SampleStore and TrackStore suffer from the same issue, which would fail similarly if constructed with a non-ResourceStore<T> store (e.g. StorageBackedResourceStore).

[peppy]

Tests need attention here.

[frenzibyte]

Turns out in master, TextureLoaderStore populates the texture extensions to game.Resources, due to...

osu-framework/osu.Framework/Game.cs

Line 181 in 6047241

addFont(localFonts, Resources, @"Fonts/Roboto/Roboto-Regular");

osu-framework/osu.Framework/Game.cs

Lines 234 to 235 in 6047241

	private void addFont(FontStore target, ResourceStore<byte[]> store, string assetName = null)
	=> target.AddTextureSource(new RawCachingGlyphStore(store, assetName, Host.CreateTextureLoaderStore(store)));

(notice Resources passed to addFont, which creates a texture loader store of it)

That is also the reason why we see lookups like file.mp3.jpg all over the place game-side.

This is relevant in the test failure because the test was never supposed to work without specifying the extension, but it was actually working because of the png/jpg extensions being added to game.Resources:

osu-framework/osu.Framework.Tests/Visual/Sprites/TestSceneTextures.cs

Lines 264 to 266 in 4fef626

	BlockingOnlineStore = new BlockingResourceStore(new NamespacedResourceStore<byte[]>(game.Resources, "Textures"));
	NormalStore = new TextureStore(host.CreateTextureLoaderStore(BlockingOnlineStore));
	LargeStore = new LargeTextureStore(host.CreateTextureLoaderStore(BlockingOnlineStore));

(notice game.Resources there in the store flow)

[frenzibyte]

I'll run tests game-side and confirm this doesn't regress skins and storyboards just to be sure.

[frenzibyte]

Can't seem to find any issues so far. This is ready for review.

[peppy]

Seems correct