New functions initialize_encoder_wic(), encode_frame_wic() and
terminate_encoder_wic() are useful for implementing GdipSaveAdd() and
GdipSaveAddImage() later.

The WIC encoder is now stored in the new GpImage "encoder" field instead
of a local variable. This makes it possible to keep the encoder active
between multiple gdiplus API calls, which is also useful for
GdipSaveAdd() and GdipSaveAddImage().
Signed-off-by: Florian Will <florian.will@gmail.com>
Signed-off-by: Vincent Povirk <vincent@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>