avcodec/tiff: Refactor TIFF tag related functions to share the code.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec: Add EXIF metadata parser to libavcodec.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mjpegdec: Read EXIF metadata in JPEG input.

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