avcodec/decode: Avoid stack packets when decoding subtitles

avcodec/decode: Avoid stack packets when decoding subtitles

Use AVCodecInternal.buffer_pkt (previously only used in
avcodec_send_packet) instead of stack packets when decoding subtitles.
Also stop sharing side-data between packets and use the user-supplied
packet directly for decoding when possible (no subtitle decoder ever
modifies the packet it is given).
Reusing AVCodecInternal.buffer_pkt is based upon an idea from James
Almer.

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>