Ensure native objects are keept alive long enough when calling native fns

[dubiousconst282]

All wrappers dealing with allocations override GC finalizers to automatically free the underlying native object as a last resort for when Dispose() fails to be called. This may cause problems in some specific circumstances:

void Problem() {
    var frame = new VideoFrame();
    ...
    encoder.SendFrame(frame);
    //No other refs to `frame`
}

public bool SendFrame(MediaFrame? frame)
{
    var result = (LavResult)ffmpeg.avcodec_send_frame(Handle, frame == null ? null : frame.Handle);
    //GC could've run just before the call and finalized `frame`, freeing the handle and possibly causing an AV
    //Calling GC.KeepAlive(frame) could've been enough to avoid this issue.
    ...