FFMPEG Plugin: Automatic detection of video decoding parameters.

- Kudos to Marcus Overhagen for laying out the general idea of automatic
  detection by sharing some of his dvb code examples with me.
- Simplify pixel format search code by removing the loop and let FFMPEG
  decide what pixel format to use based on the actual video data.
- Automatically detect the video frame rate based on the actual video data.
- Remove fOutputVideoFormat to avoid synchronizing values in two distinct
  places. The member variable fHeader is the main place for important decoder
  parameters now.
- Introduce fOutputColorSpace containing the color space that was previously
  tracked in the fOutputVideoFormat member variable.
- Update the documentation accordingly.
Media Kit: Remove superflous code in mpeg2_decoder_test.

- Thanks to automatic video decoding parameter detection, we no longer need to
  manually hack the correct parameters for this test to succeed, jiha!