rendering: add backend-neutral retained IR

[tritao]

Summary

Introduce the backend-neutral retained representation and scene traversal used by DrawList rendering.

• Add retained draw commands for geometry, transforms, material, texture, depth, blend, and raster state.
• Keep retained geometry storage independent of OpenGL resource types.
• Add `SoIRRenderAction` to translate Coin scene-graph traversal into retained commands.
• Route generic shapes through retained primitive collection.
• Preserve 1-, 2-, 3-, and 4-component texture semantics in the retained representation.
• Add focused IR tests and representative scene-graph traversal coverage.

The tests include traversal of ordinary nodes such as `SoCube` and validate emitted topology, geometry, and command ownership.

This layer does not add an OpenGL backend, GPU picking, selection rendering, render stages, or manager pipeline integration.

Stack

Layer 11/18, based on PR #59.