* range: pre/postfix with black video-frames
* fix A/V offset - decode and skip source until offset is reached
* use spinning progress-bar during decode&skip
* use video-stream index in stream-map
* fix audio-export abort; stop freewheeling
* re-indent code
* use ardour/SRC to resample audio on import
* parse framerate via codec-timebase if available
* update encoder parameters
* add vpx/webm as codec option
