Most users of ff_adts_header_parse() don't already have
an opened GetBitContext for the header, so add a convenience
function for them.
Also use a forward declaration of GetBitContext in adts_header.h
as this avoids (implicit) inclusion of get_bits.h in some of
the users that now no longer use a GetBitContext of their own.
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
