Uploads the scene ambient term (§68).
Optionalopacity: numberSelects whether this draw samples the bound albedo texture.
Uploads the frame's point and spot lights, or nothing (§68, R-17).
Switches the shadow comparison per draw (§49's receiveShadow).
Uploads the frame's shadow state, or nothing (§69, R-18).
The surface the renderer's draw loop needs from the skinned lit program —
LitProgram's contract plus the palette upload.