AVFormatContext: add metadata_header_padding field

AVFormatContext: add metadata_header_padding field

This field is used to store the number of bytes that should be written
as padding to a metadata header of a file.  For example:
 - The FLAC format's METADATA_BLOCK_PADDING [1]
 - The ID3v2 tag format's padding           [2]

[1] http://xiph.org/flac/format.html#metadata_block_padding
[2] http://id3.org/id3v2.3.0#ID3v2_overview

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