FFMPEG Plugin: Fix bug and refactor input buffer padding.

- Fixes a bug using realloc with a memory area that is declared const which
  lead to a crash in MediaPlayer playing big_buck_bunny_720p_stereo.ogg.
- The refactoring introduces a strict separation between const memory areas
  (chunk data read from GetNextChunk()) and mutable memory areas
  (fVideoChunkBuffer) by using a copy operation instead of a casted
  assignment operation.
- Updated documentation accordingly.
- Besides fixing the bug, there is no functional change intended.