apngdec: use side data to pass extradata to the decoder

Fixes remuxing apng streams coming from the apng demuxer.
This is a regression since 940b8908.

Found-by: James Almer <jamrial@gmail.com>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>