gdiplus: Don't create new instance of IWICBitmapDecoder to encode every frame of animation.

dlls/gdiplus/gdiplus_private.h

@@ -271,6 +271,7 @@ struct GpAdustableArrowCap{
 struct GpImage{
     IPicture *picture;
     IStream *stream; /* source stream */
+    IWICBitmapDecoder *decoder;
     ImageType type;
     GUID format;
     UINT flags;