OpenMW-CS: Terrain selection at the border of cells omits certain corner vertices

To reproduce:

1. Open an arbitrary cell in the scene view.
2. Under "Edit Mode" in the control bar, select the "Terrain land editing" mode.
3. Under "Change brush settings", select any brush (ideally, one with bigger dimensions).
4. Select some terrain at the corner of your current cell selection (the adjacent cell must not be loaded!).
5. Realise that the corner vertex is only selected on the NE and SW corners of the cell.

SW corner (selected)

NW corner (not selected)