avutil/buffer: add get_opaque

avutil/buffer: add get_opaque

Ported patch from ffmpeg:
    author Xidorn Quan <quanxunzhen@gmail.com>
    Fri, 29 Mar 2013 17:15:19 +0200 (23:15 +0800)
    committer Michael Niedermayer <michaelni@gmx.at>
    Fri, 29 Mar 2013 18:31:16 +0200 (17:31 +0100)
    commit c81d2fa96ddc9f6dab38d61a8803caca7e63bea0

    avutil/buffer: add get_opaque

    Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Change-Id: I456b163a92e2f87c7c4148bfb68f0f610194f3a5
Signed-off-by: Michal Jurkiewicz <m.jurkiewicz@samsung.com>
(cherry picked from commit 40bec43a69561bba5c3914d762813f32a7e1983f)