Prepare for size of struct AVPacket becoming private

[s09bQ5]

The next time the major version of libavcodec is increased, the size of struct AVPacket will no longer be part of the public API. av_init_packet and struct AVPacketList will vanish. This part of the API has been marked as deprecated in 2021.

There is a single file where we use these. Our TPacketQueue uses AVPacketList and we temporarily allocate status packets with av_malloc right before they are added to the queue.

To fix this the queue should be switched to a custom record type that contains a pointer to an AVPacket. The queue should no longer copy the packet structure and instead increase its reference count. The PutStatus method should switch to av_packet_alloc.