explicit list of major ticks should not exceed helix boundaries

[dave-doty]

Create a helix with max_offset 20, and then explicitly list major ticks at 0, 10, 20, 30, 40. Major ticks 30 and 40 are displayed even though there is no helix there. Keep the explicit list in internal storage, but display in the view only up to max_offset (and similarly cap below min_offset).