Moved creation of encoding stream wrappers to AviWriter.
Added combined interfaces IAviVideoStreamInternal and IAviAudioStreamInternal, derived IAviStreamInternal from IAviStream for ease of use.
Added base classes for wrappers over streams.
Asynchronous wrappers for streams based on Task.
Improved thread-safety in several places.
Fixed calling conventions in VfwApi.
Updated sample app to recent changes in API. Added support for MP3 encoding.