display major tick widths properly when Helix.min_offset is not 0

[dave-doty]

Major tick widths should be aligned in the center between major ticks:

However, if Helix.min_offset is not 0, they may be shifted erroneously:

[dave-doty]

There's another bug, related to this, having to do with parts of the code dealing with min offsets and z-coordinates not being treated consistently.

If you are in the none Grid, and set the z-coordinate of an individual helix to be nonzero, then the whole helix (the circle and the lines defining Helix offsets) should move left (for negative) or right (for positive) in the main view. However, they currently don't move at all.

However, keeping the z-coordinate 0 and changing the min_offset to be something else should not affect where the Helix circles are drawn, only the lines.