Model marker radius cannot be changed

[tkuchida]

Raised in Forum Topic 10568. Seems like ModelDisplayHints might be the place for this, given that it already handles marker_color. All markers would be the same size (though they're already required to be the same color).

[ajyoder]

I would re-emphasize need to add this back in. Inability to adjust model marker sizes makes it quite difficult to verify placements of markers on small bones (spine, hands), or where there are many markers close together

When making improvements, ideally users would also be able to define colors/sizes individually, such that markers could be visualized in groups

Example of a T1 vertebrae with 8 markers