FFMPEG Plugin: Implement video frame rate calculation for decoder.

- The frame rate calculation is purely based on AVCodecContext fields. This way
  the frame rate can be calculated by the FFMPEG decoder based on the actual
  video data. This is useful for automatically detection of the correct
  decoding parameters (not implemented yet but scheduled for a later commit).
- Not used anywhere yet (scheduled for a later commit).
- Also some minor documentation updates.
FFMPEG Plugin: Automatic detection of video decoding parameters.

- 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!