backdrop
Builds a group holding one full-bleed color-filled rectangle over bounds — a prebuilt scene backdrop, usable directly as a placeholder scene or nested as a scene's first child.
The rectangle is an origin-centered width × height quad, ignoring SceneBounds.minX and SceneBounds.minY — the renderers' offset-ignoring clip convention — so it exactly covers what renders. It is fill-only: color fills, no stroke.
Return
The one-child GroupNode.
Parameters
bounds
The viewport to cover; also set as the group's own GroupNode.bounds.
color
The fill, opaque black by default.