transparency: extra depth buffer

[toloudis]

Use Case

When PDB geometry is rendered, there are so many overlapping spheres that the alphas accumulate to essentially be totally opaque. They don't look transparent. This can happen with any type of geometry but PDBs are easy to see the problem.

Acceptance Criteria

We want roughly the frontmost faces of an agent to show transparency and hide the complexity of the possible transparent layers behind it. At least on a per agent basis but possibly globally.

Details

Consider implementing a second depth buffer to handle transparency, and then generate a color buffer to be blended with the regular opaque geometry as an overlay.

[toloudis]

Depends on results of the easier-to-implement #309
This ticket probably merits a meeting to discuss and brainstorm further.