dvb.media_addon: Code cleanup and some coding style.

- Remove use of now superflous media_header_ex structure.

- Respect 80 char line limit and indentation rules. Coding style only applied
  to functions raw_[audio|video]_thread, rest of the class may need some more
  cleanup.

- No functional change intended.
dvb.media_addon: Trigger loading of media plugins.

- This is the workaround as described in the bug report #11018.
- I would have preferred to place this workaround in DVBMediaAddon::InitCheck()
  as this method can return a status code but unfortunately it is never called.
dvb.media_addon: Remove superflous init check.

- This init check prevented the MediaStreamDecoder from setting up an input
  format. The values the init check is based on are initialized by the first
  call to BMediaDecoder::SetTo() as described in the BeBook ("If you use the
  empty form of the constructor, you'll have to call SetTo() to establish the
  format to be decoded before calling Decode()"). But MediaStreamDecoder
  doesn't provide a public way to call BMediaDecoder::SetTo() first.
  There are at least two possible solutions to remedy this situation:

  1. Remove the init check
  2. Add a public way to call SetTo()

  The first solution was chosen for the following reason: Least amount of
  code change needed as it is an implementation detail so no client code has
  to be touched.
  The second solution might make sence when you want to stick to the BeBook
  where a clear distinction between SetTo() and SetInputFormat() is made, but
  as the current MediaStreamDecoder implementation doesn't stick to the BeBook
  anyway there is less need to add another method at this time.
dvb.media_addon: Stop changing format on every video frame.

- Due to comparing the wrong fields with one another the dvb.media_addon called
  ChangeFormat() on every new video frame.
- Also remove a duplicate print statement.
FFMPEG Plugin: Calculate media_header.size_used field

- This field is needed by the dvb.media_addon to detect a format change so
  fill it with the correct value.
- Updated documentation accordingly to reflect reordered function calls and new
  assumptions based on the function call ordering.
FFMPEG Plugin: Fix video start_time handling

- I misinterpreted the semantics of reordered_opaque. I thought it would
  establish the correct relationship between the start_time returned by
  GetNextChunk() and the next video frame successfully decoded. But for this
  to work reordered_opaque expects to be filled with presentation time stamps.
  A series of presentation time stamps may be jumping back in time due to the
  presence of B-frames. The decoded frame presentation time stamps series would
  then be ordered in a monotonically increased way.
  But actually GetNextChunk() always returns monotonically increasing start
  times. Mapping this behaviour to FFMPEG's expectations means labeling those
  start times as decoding time stamps (dts). Though for those start times to be
  related to the correct decoded video frames you have to assign the start time
  with the AVPacket containing the data to be decoded.

- This commit finally makes DVB video playback working for me with the TV app.
  Though no audio yet.

- The documentation was updated accordingly.
FFMPEG Plugin: Fix video start_time generation.

- Ensure that start times are increased monotonically for video formats that
  contain B-frames, too, as expected by the BMediaDecoders.
  Previously start times were returned that seemed to go back in time for
  videos containing B-frames.
  Tested with resolutionchange.mpg (\see http://samples.ffmpeg.org/MPEG2)
  Note: Even though start times aren't going back in time anymore there are
  times where two consecutive start times are equal. This would need more
  research once this exposes a bug in a real application. Further more this
  might seem like a new bug, but before this commit the equal start times would
  have simply some different start time[s] in between. So at most this is the
  same bug just wearing new clothes :)

- Documentation updated accordingly.