rendering: integrate DrawList with SoRenderManager

[tritao]

Builds on #52.

Purpose

The retained renderer is not useful to applications until it can be selected
and driven through Coin's existing render-manager APIs. This PR connects the
DrawList pipeline to SoRenderManager while keeping renderer implementation
details and application interaction policy out of the manager.

What changed

• Add DrawList pipeline selection and backend creation to SoRenderManager.
• Forward the camera, viewport, and device-pixel ratio into retained traversal
  and execution.
• Preserve manager callbacks and render-root orchestration.
• Replace or release backend resources deterministically while the current
  OpenGL context is valid.
• Expose resolved retained picking through the manager integration surface.

Ownership and scope

The manager chooses and orchestrates a pipeline. It does not define what
selection means or own long-lived application identity. Selection and
highlight targets remain frame-local renderer inputs; applications retain
their own policy and object model.

Render layers, delayed replay, and context-loss lifecycle are added in #55.

Validation

• Manager-driven DrawList rendering.
• Camera, viewport, DPR, callback, and fallback contracts.
• Backend replacement and cleanup with a valid context.
• Manager-facing picking and frame-local selection identity.