We've added some classes that make it possible to display 3D grids and continuous space in 2D, by projecting along the Z dimension. They're in sim/portrayal/grid and sim/portrayal/continuous. The manual has been updated to discuss them (Section 18.104.22.168).
One side effect of adding these classes is that I needed to move StableInt3D and StableDouble3D from sim/portrayal3d/inspector/ to sim/portrayal/inspector. That might *possibly* require you to change an include statement in your code (probably not). And as there's nothing left in sim/portrayal3d/inspector, it has been deleted.
These classes are entirely untested.