Explicit background for passes that clear the frame (e.g. RenderPass and
the OIT opaque clear). When set, the pass clears to this colour/alpha instead of
the renderer's current clear state, giving a single source of truth so the
background is identical regardless of which base pass renders the scene. When
null the pass falls back to the renderer's current clear colour/alpha.
Terminal pass: composites the pipeline's linear HDR buffer to the screen and applies tone mapping + sRGB output encoding once, for the whole buffer, via
renderer.toneMapping/toneMappingExposure(toneMapped: truebelow).Because tone mapping is deferred to this single pass, scene materials must render linear (
toneMapped: false) or they double-tone-map, and the per-materialtoneMapped = falseopt-out is not honored. See documents/oit-guide.md ยง7.