Revert "apngdec: use side data to pass extradata to the decoder"

This reverts commit e0c6b32046f4bab7d34be77dd2f03b2a80c86d39.

Said commit changed the behavior of the demuxer and decoder in a non
backwards compatible way.
Demuxers should make extradata available at init if possible, and send
new extradata as side data within a packet if needed.

A better fix for the remuxing crash will follow.

Signed-off-by: James Almer <jamrial@gmail.com>
