avformat/http: support reading compressed data

Derived from VLC's http module.
Original authors:
  Antoine Cellerier <dionoea@videolan.org>
  Sébastien Escudier <sebastien-devel@celeos.eu>
  Rémi Duraffort <ivoire@videolan.org>
  Rémi Denis-Courmont <remi@remlab.net>
  Francois Cartegnie <fcvlcdev@free.fr>

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