demux: mp4: point to atom's position of base offset is moov (fix #11746)
demux: mp4: point to atom's position of base offset is moov (fix #11746)

Not so common flags, lack of samples.
Unsure if it doesn't break something else.

(cherry picked from commit 4fe0751b76f43b9b0ac6aab34319a94e498536cd)
Signed-off-by: Jean-Baptiste Kempf <7b85a41a628204b76aba4326273a3ccc74bd009a@videolan.org>

jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2