Update track-container.js

When you resize a video track in the track-container area making it shorter/longer from the left side there is a bug. It is not possible to shorten a video track from the beginning it always does from the end.The same thing happens when you try to make it longer.

Now it is possible to shorten/lengthen the video from both sides :

 if (trackEvent.popcornOptions.start - newStart > 0 ) sign=sign*(-1);
          popcornOptions.from = Math.abs(
            Math.abs(trackEvent.popcornOptions.start  - (trackEventView.element.offsetLeft / _container.clientWidth * _media.duration))
            + (trackEvent.popcornOptions.from*sign)
          );