Apply mode's pick-pass consequences. Both are RUNTIME uniforms, never build
flags, so a layers-panel mode switch does not recompile the pick program.
Match the visual material's face culling.
Grouped with setPickMode rather than given its own capability because both are
"copy the visual material's current epoch state", both are driven from the same
one place (PickingSystem.renderPickBuffer), and both are mesh-only — a second
guard would buy nothing but a second thing to forget.
A pick material that tracks the visual material's whole per-epoch appearance state, not just a single boolean. Implemented by both mesh pick wrappers, and by nothing else — the two members below are the two things a mesh pick pass must copy from its visual twin that the sibling pick materials do not.