Make pseudobonds end on cartoons

[Eric Pettersen]

Pseudobonds to backbone atoms that are hidden by cartoon end in space and should instead end on the cartoon.

Pseudobonds to backbone atoms are hidden by cartoons. They need to be shown. In 1www for example as soon as you turn on cartoons, the missing-structure pseudobonds disappear. Also, pseudobonds depicting hydrogen bonds and distances to backbone atoms need to show up. They probably need to go to a "corrected" position so that there's no huge gap between their endpoint and the cartoon. I guess that's a discussion topic since you _could_ leave the gap and inform users they need to use a spline that goes exactly through the atom positions. That latter approach is unappealing since I envision having to say that basically 10,000 times over the lifetime of ChimeraX to various users.

Set the owner to T.G. for the "show pbs to hidden atoms" part, which would switch to C.H. for the "position adjustment maybe" part.

[Tom Goddard]

It is a few line change to show pseudobonds to hidden atoms (add C++ PBond::shown that checks atom "display" instead of "visible"). But that will make these pseudobonds end in empty space. That doesn't seem like the right solution.

We are using tethers for showing the association of atoms to a ribbon. We need a strategy for showing pseudobonds that end on hidden ribbon atoms. Since Conrad is handling ribbons I'm reassigning to him to figure out the desired solution.

[Tom Goddard]

About a month ago I made pseudobonds to hidden atoms visible. Now the trouble is the missing segment pseudobonds end at a point in space instead of on the cartoon since the cartoon does not pass exactly through the hidden backbone atoms.