Fix a timestamp rounding issue in `FFmpegFrameGrabber` that causes `setFrameNumber()` to sometimes pick the wrong frame if FPS is not a proper divisor of 1000000 (issue #5)
Merge pull request #6 from juergenuhl/master

 * Fix a timestamp rounding issue in `FFmpegFrameGrabber` that causes `setFrameNumber()` to sometimes pick the wrong frame if FPS is not a proper divisor of 1000000 (issue #5)