FFMPEG plugin: Use member variables instead of local ones.

- This small refactoring is in preparation for implementing decoding of partial
  video frame data, where one needs to preserve encoded data between multiple
  calls to  AVCodecDecoder::_DecodeVideo().

- Note: The names fChunkBuffer and fChunkBufferSize are open for discussion.
  I'd rather prefer fEncodedDataChunkBuffer and fEncodedDataChunkBufferSize.
  But I'd like to take small refactoring steps, and changing the naming would
  also touch the AVCodecDecoder::_DecodeAudio() function. I'd rather focus
  on improving the video part for now, leaving the audio part alone.

- No functional change intended.

Signed-off-by: Colin Günther <coling@gmx.de>
(cherry picked from commit f2da1e752458b926aebe50642bf6af19e9903f17)
FFMPEG plugin: Add links to useful FFMPEG-API documentation.

- As it is rather hard to find useful documentation, on how to use FFMPEG's API
  we cut of some research time by just documenting it at the place where it is
  most visible to other FFMPEG plugin developers :)

Signed-off-by: Colin Günther <coling@gmx.de>
(cherry picked from commit b0ed15d2196ca45de8ab2d5913d760602aacc5f3)
FFMPEG plugin: Refactor video decoding function.

- We factor out the code that does the decoding, deinterlacing and color space
  converting into its own function. This prepares auto detection of video frame
  properties in a later commit. Auto detection means (for example), that you
  don't need to know the size of the decoded video frame -before- intitializing
  the video codec as it is the case with the current implementation.

- Use the already existent private member variable fHeader to fill out and
  return video frame properties.

- Rename some variables to be more self describing.

- No functional change intended.

Signed-off-by: Colin Günther <coling@gmx.de>
(cherry picked from commit 6beab04f48e98022041895750f38c5a35a5f4f9e)