Miscellaneous fixes to handle WebM files in the wild

- Make EBML header validation output useful error messages.
- Fix ebml.IdToName() to return a helpful string for unknown IDs.
- Created ebml.Filter() and webm.Filter() functions to make it easy to filter
  out unwanted elements from a buffer.
- Changed mse_webm_remuxer to filter out deprecated FrameRate elements.
- Changed mse_webm_remuxer to fix Vorbis SimpleBlock keyframe flag if isn't set.
- Added support for default timecodeScale to webm.InfoParser.