FFMPEG Plugin: Implement video input buffer padding.

- Padding is required by FFMPEG for correct operation of all video decoders.
  FFMPEG performs some speed optimizations under the hood that may lead to
  reading over the end of the chunk buffer wouldn't there have been padding
  applied.
- Note: Padding is needed for audio decoders, too. Will tackle this in some
  later commits. For the time being we have a degradation in code reuse, due to
  different memory ownership of chunk buffers in audio and video decoder path.
  Audio path must not care about freeing chunk buffers whereas video path must.
- Fix coding style and some typos.
- Update documentation accordingly.