obs-ffmpeg: Use av_packet_alloc instead of av_init_packet

sizeof(AVPacket) being a part of the public ABI is deprecated. once
av_init_packet() is removed, new packets will only be able to be
allocated with av_packet_alloc().
In ffmpeg-mux, ffmpeg-vaapi, and obs-ffmpeg-nvenc, AVPacket is allocated
at the initialization to avoid frequent allocation of AVPacket.

Includes changes to win-dshow.