Support rerendering on scrolling

[raczben]

Left-right scrolling does not update and re-render the waveform.

The root cause is that zoom_fast() fires a scroll event. Therefore scrolled() called during each zoom_fast event. So scrolled should and should not contain the re-render function.

The scrolled() function should distinguish between "original-scroll" (the render should called), and the "zoom-scroll".