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 required for audio decoders, too. I 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.
FFMPEG Plugin: Refactor out loading next video chunk.

- Main reason for refactoring was to increase readability of
  _DecodeNextVideoFrame() by simplifying it. Refactoring was tested
  successfully for no functional change with mpeg2_decoder_test.
- Reindented the method definition in the header file so that the new method
  _LoadNextVideoChunkIfNeededAndUpdateStartTime() fits into 80 chars per line.
  Reindentdation is applied to methods only as the member variables have no
  space left for reindentation.
- Update documentation accordingly.
- Fix wording of audio part to audio path.
- No functional change intended.
FFMPEG Plugin: Fix bug and refactor input buffer padding.

- Fixes a bug using realloc with a memory area that is declared const which
  lead to a crash in MediaPlayer playing big_buck_bunny_720p_stereo.ogg.
- The refactoring introduces a strict separation between const memory areas
  (chunk data read from GetNextChunk()) and mutable memory areas
  (fVideoChunkBuffer) by using a copy operation instead of a casted
  assignment operation.
- Updated documentation accordingly.
- Besides fixing the bug, there is no functional change intended.