Move marker visuals 0.2mm forwards to prevent z-fighting

[Adimote]

Move the visual element of a marker forwards by 0.2mm to prevent z-fighting in the recordings. This does not change the detections of the markers at all

Before:

https://user-images.githubusercontent.com/2551763/221275105-136ec474-3d69-453c-9c94-1268e49b7b92.mp4

After:

https://user-images.githubusercontent.com/2551763/221275121-28817437-dbb5-4c9c-9e70-26adc22e5ef5.mp4

fixes #387

[PeterJCLaw]

@Adimote do you think we should merge this? Clearly we managed without it, so the issue can't have been too bad.

Separately: would a 1mm shift be sufficient? Feels like that would be visually preferable if so.