Screen visualization incorrect for image at origin

[rikvl]

In the example examples/two_screens.py, the following cross product doesn't work for the direct-line-of-sight image [the one with pos = (0, 0, 0)].

screens/examples/two_screens.py

Line 75 in ce47d9b

upos = pos + (ZHAT.cross(unit_vector(pos))

As a result, the line associated with this image is not drawn perpendicular to the screen's normal (line of lensed images).