Fix rendering of vertexes of shape layers with small scale

[Czaki]

References and relevant issues

Bug spotted in context #6627

Description

After changes in #5802 the vertex border is very thick:

This PR reverts this change as Marker's size is in absolute unit, not data unit.

Script for reproduce:

import napari
import numpy as np


s = (0.01, 0.005, 0.005)
s = (0.005, 0.005, 0.005)
s = (0.01, 0.01, 0.01)
s = (0.1, 0.1, 0.1)
# s = (1, 1, 1)
viewer = napari.Viewer()
layers = viewer.open_sample("napari", "cells3d")
for l in layers:
    l.scale = s

shapes = viewer.add_shapes(
    np.array(
        [
            [29.0, 130.70802005, 86.37901572],
            [29.0, 130.70802005, 137.70733652],
            [29.0, 182.32797904, 137.70733652],
            [29.0, 182.32797904, 86.37901572],
        ]
    ),
    scale=s,
)
shapes.mode = "select"
viewer.dims.current_step = (29, 0, 0)
viewer.reset_view()

napari.run()

[brisvag]

Nice detective work! This code is all scattered around and it was hard to follow, it's a pity it slipped through :/

Good that this works still with anisotropic scales!