lavc/exif: Make EXIF IFD decoding part of private API/ABI.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavf/avidec: Read metadata EXIF tags from AVIF tag. Based on patch by Gregory Wolfe (Kodak Alaris) <gregory.wolfe@kodakalaris.com>.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Reindent after last commit.

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