Show selected atoms under transparent surface

[tic20@…]

Current selection outline code does not associate a depth with the outline and will only show an outline for frontmost objects.

Maybe a better solution is to draw an outline for opaque objects with and outline depth, and a separate outline for transparent objects.

Need to think about the case where a selected atom is behind a selected transparent surface. Should both be highlighted? Currently selection outlines are only at boundaries between unselected and selected items (which would result in no outline for the atom).

If a selected transparent surface is inside another selected transparent surface should both show outlines? I think currently only one transparent layer is shown, so the inside object would not even be visible.

[Tom Goddard]

Fixed.

The selection outlines for objects behind transparent surfaces are not dimmed by the overlying surface, they are full brightness. This was easier. Maybe it is also better so that the outline is easy to see.

[Eric Pettersen]

Milestone renamed