Fix bug on vflip filter

[SIY1121]

Fixed a bug on using vfilter.

[saudet]

I think the correct fix for that would be to change frame.imageStride = filt_frame.linesize(0) to frame.imageStride = Math.abs(filt_frame.linesize(0)). Though, does that work?

[SIY1121]

You have got a point.

In fact, I corrected frame.imageStride = filt_frame.linesize(0) to frame.imageStride = Math.abs(filt_frame.linesize(0)) first. But it didn't work.

To be exact, jvm crashed when I pushed the frame to FFmpegFrameRecorder.
Here is a simple code I tested:

        FFmpegFrameGrabber grabber = new FFmpegFrameGrabber("in.mp4");
        grabber.start();

        FFmpegFrameFilter filter = new FFmpegFrameFilter("vflip", grabber.getImageWidth(), grabber.getImageHeight());
        filter.start();

        FFmpegFrameRecorder recorder = new FFmpegFrameRecorder(new File("out.mp4"), grabber.getImageWidth(), grabber.getImageHeight());
        recorder.setFrameRate(grabber.getFrameRate());
        recorder.start();

        while (true) {
            Frame frame = grabber.grabImage();
            if (frame == null) break;
            filter.push(frame);
            recorder.record(filter.pull());
        }

        grabber.stop();
        filter.stop();
        recorder.stop();

and error log:
hs_err_pid6300.log

First of all, I think behavior of returning a negative stride is not bug on ffmpeg.
I found a article about the negative stride.

I'm not good at ffmpeg, but making image stride negative seems to be one of vflip processes.
Also I think you musn't modify frame data from outside ffmpeg because it can cause crashes like above.