Refactor:

It seems to be no big deal to have a lot of small files; I think it's the
actual scan of the song tree that's expensive.  Simplify with this in
mind.

Load song banners when the song is loaded, so we don't load banners
for songs that aren't there.

Avoid touching the original banner when we're simply loading.

stepmania/src/FadingBanner.cpp

@@ -99,7 +99,7 @@ bool FadingBanner::LoadFromCachedBanner( const CString &path )
 	}
 
 	/* It's not loaded.  Try to load the low quality version. */
-	RageTextureID ID = BANNERCACHE->LoadCachedSongBanner( path );
+	RageTextureID ID = BANNERCACHE->LoadCachedBanner( path );
 	if( !TEXTUREMAN->IsTextureRegistered(ID) )
 		return false;