avformat/movenc: use correct iTunes copyright atom

Support for writing copyright metadata was added in commit bed4fc54c9
for 3GP, MOV and iTunes metadata. 3GP and MOV cases are formally
specified. However, iTunes format does not have specification, and
it seems to have been assumed that it would use the same atom as
MOV (both being Apple formats).

However, Apple uses 'cprt' atom for iTunes metadata (do note that
the iTunes 'cprt' encoding is generic iTunes ItemList atom, not
the 3GP 'cprt' encoding. These are also inside different parent
atoms).

Most references trying to document iTunes atoms mention only
the 'cprt' tag. See:
 - http://atomicparsley.sourceforge.net/mpeg-4files.html
 - http://mutagen.readthedocs.io/en/latest/api/mp4.html

Same applies to other software supporting this tag. Most of them
encode and decode only the 'cprt' atom.

ffmpeg mov demuxer supports both atoms in this context. There are
few pieces of other software that support similarly both 'cprt' and
the incorrect '\251cpy' atom in this context. I believe they do it in
order to read the ffmpeg encoded incorrect copyright atom.

In light of the above this changes the copyright atom to 'cprt' as
it seems to be supported univerally and is the correct atom to use.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

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