this ensures that the path is a closed polygon.
this matters for PenCap and PenJoin purposes.

test_PolylineROI has the following series of test images:

1. init
2. hover_roi
3. drag_roi
4. hover_handle
5. drag_handle
6. hover_segment
7. click_segment
8. drag_new_handle
9. clear

Prior to this PR, aarch64 (and ppc64le) platforms fail at init. (See #2110)
With this PR, the failure occurs later at click_segment (and drag_new_handle)

When the path is not closed, we end up with a polyline with its ends rendered with SquareCap, which according to Qt docs: "a square line end that covers the end point and extends beyond it by half the line width."
It does so happen that the failing pixels in init correspond to the closing point of the ROI handle.

Failure screenshot for click_segment. For this case, a lot more pixels are involved.