This improves code readability and this avoid warnings about discarding
qualifiers from pointer target type.
Originally committed as revision 16952 to svn://svn.ffmpeg.org/ffmpeg/trunk
metadata using old API, it is automatically converted to new API before
muxing.
Originally committed as revision 16447 to svn://svn.ffmpeg.org/ffmpeg/trunk
avi is updated as example.
No version bump, the API still might change slightly ...
No update to ffmpeg.c as requested by aurel.
Originally committed as revision 16424 to svn://svn.ffmpeg.org/ffmpeg/trunk