tiff: fix handling of metadata with refcounted frames

Since the conversion to refcounted frames, the tiff decoder
only wrote the metadata into its internal "picture" in its own context,
never exposing the metadata to the user, and eventually leaking the
metadata.

Instead, properly store the metadata directly into the frame being decoded into.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
