avformat: Add and use helper function to add attachment streams

avformat: Add and use helper function to add attachment streams

All instances of adding attached pictures to a stream or adding
a stream and an attached packet to said stream have several things
in common like setting the index and flags of the packet, setting
the stream disposition etc. This commit therefore factors this out.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>