Fix deprecation warnings

[Kagami]

As per documentation:

• AVCodecContext.codec should be replaced with codecpar
• avcodec_decode_video2 should be replaced with avcodec_send_packet/avcodec_receive_frame

And few more fixes:

• Don't allocate frame for each packet
• Fallback to default decoder if libvpx(-vp9) not available

[bakape]

Thanks! This however will no longer compile with 3.0, so a version bump will be needed.