opusenc: Handle LUFS reference loudness in FLAC

Handle the FLAC comment REPLAYGAIN_REFERENCE_LOUDNESS expressed in
either dB SPL (positive) or LUFS (negative).  Although the FLAC
reference software writes dB SPL, some other software (such as
loudgain -s e) writes LUFS.

Fixes xiph/opus-tools#2316
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