/**
* calculate positions markers on the alignment ruler
*
+ * @param av
+ * @param startx
+ * left-most column in visible view
+ * @param endx
+ * - right-most column in visible view
* @return List { Object { .. } } Boolean: true/false for major/minor mark,
* Integer: marker position in alignment column coords, String: null
* or a String to be rendered at the position.