FFMPEG Plugin: Print debug output for audio frames, too

- Also change what is printed for video frames. Currently both
  debug_fframe_[audio|video] are used in AVCodecDecoder only and thus are
  streamlined for their usage there. For example we print the AVFrame.pkt_dts
  field instead of the AVFrame.pkt field because the later one is never touched
  by AVCodecDecoders usage of the FFMPEG library.
  Note: AVFrame.pkt being never touched means that it always contains the value
  AV_NOPTS_VALUE making it less useful for debug purposes.
FFMPEG Plugin: Small cleanup

- The reason for compiler complaining about "INT64_C is not defined here" is
  gone since the addition of the compiler flag "-D__STDC_CONSTANT_MACROS"
  to the Jamfile some time ago. This flag allows C++ to use C99 math features.

- No functional change intended.
FFMPEG Plugin: Refactor start time handling in audio path.

- FFMPEG handles the relationship of start time between encoded and decoded
  audio data now by using the fTempPacket->dts and the
  fDecodedDataBuffer->pkt_dts fields. We still have to manually keep track of
  start times for consecutive audio frames though to support returning a number
  of audio frames that may assembled of partial AVFrames.

- The start time of the very first audio frame data packet returned by Decode()
  is now correctly calculated based on GetNextChunk() start times instead of
  being always zero.

- Introduce fRawDecodedAudio that serves as a container to store properties of
  the audio frames stored in fDecodedData. This prepares the population of the
  fHeader structure with audio frame properties needed to allow clients of
  BMediaDecoder::Decode() detect audio format changes in a later commit.

- Remove fStartTime as it is superflous now.