MediaKit: Extend media_[audio|video]_header structures.

- This allows a BMediaDecoder (e.g. FFMPEG Plugin) to communicate back format
  changes to its clients.
  For a more thorough explanation and discussion see haiku-development mailing
  list: http://www.freelists.org/post/haiku-development/Request-for-protest-Media-Kit-Extend-media-header-struct

- Backwards compatibility is taken into account and preserved by reducing the
  relevant _reserved[] fields.

- Code changes that will actually make use of these extensions are due for the
  following commits. As these structure extensions affect several Haiku
  components (e.g. media_server, MediaPlayer, etc.) and third party apps (e.g.
  StampTV, etc.) I refrain from committing them in one batch with this commit.
  This should make it easier to track down bugs originating in this code
  change.
(cherry picked from commit 806b6888d2dcf84b4934f8f137a48d3381864d1c)