OpenMW-CS: Circle brush in "Terrain land editing" mode sometimes includes vertices outside its radius

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 the circle brush.
4. Choose an even value for the "Brush size".
5. Select some terrain.
6. There are vertices outside the brush's radius.