* fix race-condition & oow in extracted file output
* add mutex for file.Write + set finalizer for os.File
* fix integration test
* disable extractor save to file in lib mode(configurable)
* use sync.Once for init
* disable out of bound image write in headless
* misc updates
* fix headless screenshot test
* fix extractor save to file integration test
* remove 'to' feature in extractors