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>
lavf/flacenc: use metadata_header_padding

Allows a user to control the amount, if any, of padding they want added
to the file.  If set to zero the block will not be written at all.  If
set to some positive number four more bytes will be added to the file
due to the small header required for the block.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavf/flacenc: fix comment after previous change

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavf/id3v2enc: use metadata_header_padding

As with the change to flacenc this allows the user to control the amount
of padding they want added to the file.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavf/id3v2enc: update comment about minimum padding

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavf/avienc: use metadata_header_padding

The muxer will write at least the number of bytes requested and possibly
up to 3 bytes more.  This is because the muxer writes 32-bit integers
and the format requires 4-byte alignment anyway.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavf/avienc: cosmetic indent

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