partially revert previous commit: make the mp4box object global for debug
partially revert previous commit: make the mp4box object global for debug

sjames1958gm/mp4box.js,cconcolato/mp4box.js,b123400/mp4box.js,hanzeil/mp4box.js,b123400/mp4box.js,gpac/mp4box.js,gpac/mp4box.js,hanzeil/mp4box.js,sjames1958gm/mp4box.js,cconcolato/mp4box.js