Avoid seeking too close to the end of the stream

This can throw off delay predictions.

Hidden frames don't make it into the delayed picture buffer,
so the distance needs to be computed in terms of real frames.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

src/core/videosource.cpp

@@ -811,6 +811,13 @@ bool FFMS_VideoSource::SeekTo(int n, int SeekOffset) {
             throw FFMS_Exception(FFMS_ERROR_SEEKING, FFMS_ERROR_UNKNOWN,
                 "Frame accurate seeking is not possible in this file");
 
+        // Seeking too close to the end of the stream can result in a different decoder delay since
+        // frames are returned as soon as draining starts, so avoid this to keep the delay predictable.
+        // Is the +1 necessary here? Not sure, but let's keep it to be safe.
+        int EndOfStreamDist = CodecContext->has_b_frames + 1;
+
+        TargetFrame = std::min(TargetFrame, Frames.RealFrameNumber(std::max(0, VP.NumFrames - 1 - EndOfStreamDist)));
+
         if (SeekMode < 3)
             TargetFrame = Frames.FindClosestVideoKeyFrame(TargetFrame);