start, end and line segmentation marks

When switching to bw images, please change the default markup colors used for beginning and baseline segmentation to bright red like teachers in the good old time. The difference between purple and black is too straining for the eye to find mistakes, e.g. . In principle this goes also for the color images. We should probably also test wider lines and other colors there. Controlling the end of lines is often very difficult, because the difference between purple/dark-blue and the black or brown ink.