rendering: integrate DrawList with SoRenderManager

[tritao]

Summary

Integrate the retained DrawList renderer with Coin's existing render managers for ordinary single-scene rendering.

• Add explicit `LEGACY_GL` and `DRAW_LIST` render-pipeline selection.
• Use DrawList as the available pipeline when LegacyGL is not built.
• Give `SoRenderManager` ownership of the retained traversal action and render backend.
• Reinitialize retained backend state when the active GL context changes.
• Propagate the manager's real camera state and effective view/projection matrices into retained frames.
• Forward device-pixel ratio into retained render parameters.
• Preserve ordinary pre-render and post-render callback behavior exactly once per frame.
• Forward pipeline selection through `SoSceneManager`.
• Add manager-driven DrawList coverage for pipeline selection, camera matrices, DPR, callback counts, and core-profile rendering.

This layer intentionally handles one ordinary scene only. Background/foreground roots, additional render stages, layer-local viewport behavior, and context-loss recovery are introduced by PR #45.

Stack

Layer 15/16, based on PR #31.