Delay writing image cache index until after load complete

While loading all songs, disable writing the index file for the image
cache, in order to speed up loading and cache generation, similar to
how the song cache index file write is delayed.

src/SongManager.cpp

@@ -158,7 +158,8 @@ void SongManager::InitSongsFromDisk( LoadingWindow *ld )
 	RageTimer tm;
 	// Tell SONGINDEX to not write the cache index file every time a song adds
 	// an entry. -Kyz
-	SONGINDEX->delay_save_cache= true;
+	SONGINDEX->delay_save_cache = true;
+	IMAGECACHE->delay_save_cache = true;
 	LoadStepManiaSongDir( SpecialFiles::SONGS_DIR, ld );
 
 	const bool bOldVal = PREFSMAN->m_bFastLoad;
@@ -168,6 +169,8 @@ void SongManager::InitSongsFromDisk( LoadingWindow *ld )
 	LoadEnabledSongsFromPref();
 	SONGINDEX->SaveCacheIndex();
 	SONGINDEX->delay_save_cache= false;
+	IMAGECACHE->WriteToDisk();
+	IMAGECACHE->delay_save_cache = false;
 
 	LOG->Trace( "Found %d songs in %f seconds.", (int)m_pSongs.size(), tm.GetDeltaTime() );
 }