Strokes: Error drawing count when verts are occluded

[jlampel]

To reproduce, draw a stroke and then rotate the view so that the selected verts are occluded by the mesh.

Traceback (most recent call last):
  File "C:\Users\Jonathan\AppData\Roaming\Blender Foundation\Blender\4.3\scripts\addons\retopoflow\retopoflow\rfoverlays\loopstrip_selection_overlay.py", line 101, in draw_postpixel_overlay
    lbl_pos = get_label_pos(context, lbl, boundary)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Jonathan\AppData\Roaming\Blender Foundation\Blender\4.3\scripts\addons\retopoflow\retopoflow\rfoverlays\loopstrip_selection_overlay.py", line 50, in get_label_pos
    mid = sum(boundary, Vector((0,0,0))) / len(boundary)
          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
ZeroDivisionError: Vector division: divide by zero error