avformat/segafilm - fix keyframe detection and set packet flags

Streams from a Segafilm cpk file can't be streamcopied because
keyframe flag isn't correctly set in stream index and
said flag is never conveyed to the packet

Fixes #7091

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>