+ Single background image, done by Scalebar track, dynamically extends to hull browser height
+ image_container has .loading class while data is being rendered (semi-transparent grey bg)
+ tracks are now rendered independently, each one tracks it's own dataRange and imgRange, loads data and renders when necessary
+ new parameters for tracks:
+   buffer: number of widths to draw
+   dataBuffer: number of basepairs, to add to each end of getData request (useful for DAS transcript)
+ scrollContainer (of class .scroll_container): new track element, to which image containers are added for scrolling purpose
+ no more adjusting the left position of static elements inside the track container
- Track.drawBackground is temporarily disabled, Scalebar takes care of default background for the entire browser