fix: Make sure the decoder options isn't freed

[hfiguiere]

• This caused the decoding options to not work as intended
• valgrind was unhappy

Without the as_ref(), the decoding_options gets dropped, dropping the allocated inner. Then the values inside become garbage, including ignore_transformations.

Note: there is no testing of this in the teste suite. I couldn't figure out how make a proper sample as merely changing the orientation with ExifTool isn't enough. If I figure it out, I can submit a PR testing ignore_transformations.