cursor scrolling should be sensitive to bar lines

[aspiers]

When the scrolling cursor reaches the right-hand scrolling margin, it will jump back to the left hand side. Typically this results in the left side of the video frame showing a partial bar. It would be nicer if the cursor could jump back earlier, at the end of a bar, so that the left side of the video frame almost always starts on a bar line. "Almost" because this is not possible with very wide bars.