mp4: Simplify and speed up metadata editing.

mp4: Simplify and speed up metadata editing.

Currently the aac audio format handler first calls mp4_open_meta()
to get the metadata tags, then alters the in-memory structure of the
tags according to the command line options and passes this modified
structure to mp4_meta_update() to rewrite the tags. This latter call
parses the tags again, which is unnecessary overhead.

This patch changes the signature of mp4_meta_update() to accept an
mp4 structure instead of a callback structure and uses that instead
of re-opening the file.