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/ImageCache.cpp

@@ -169,6 +169,7 @@ void ImageCache::UnloadAllImages()
 }
 
 ImageCache::ImageCache()
+	: delay_save_cache(false)
 {
 	ReadFromDisk();
 }
@@ -457,9 +458,16 @@ void ImageCache::CacheImageInternal( RString sImageDir, RString sImagePath )
 	ImageData.SetValue( sImagePath, "Width", iSourceWidth );
 	ImageData.SetValue( sImagePath, "Height", iSourceHeight );
 	ImageData.SetValue( sImagePath, "FullHash", GetHashForFile( sImagePath ) );
-	ImageData.WriteFile( IMAGE_CACHE_INDEX );
+	if (!delay_save_cache)
+		WriteToDisk();
 }
 
+void ImageCache::WriteToDisk()
+{
+	ImageData.WriteFile(IMAGE_CACHE_INDEX);
+}
+
+
 /*
  * (c) 2003 Glenn Maynard
  * All rights reserved.