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.h

@@ -13,6 +13,7 @@ public:
 	ImageCache();
 	~ImageCache();
 	void ReadFromDisk();
+	void WriteToDisk();
 
 	RageTextureID LoadCachedImage( RString sImageDir, RString sImagePath );
 	void CacheImage( RString sImageDir, RString sImagePath );
@@ -23,6 +24,8 @@ public:
 
 	void OutputStats() const;
 
+	bool delay_save_cache;
+
 private:
 	static RString GetImageCachePath( RString sImageDir, RString sImagePath );
 	void UnloadAllImages();