Add retained draw-list rendering pipeline with modern OpenGL backend

[tritao]

This PR introduces a retained draw-list rendering pipeline alongside Coin’s existing legacy OpenGL renderer.

The work adds a render intermediate representation, a modern OpenGL backend, retained traversal support for core scene-graph nodes, GPU picking, layered rendering, and integration with SoRenderManager.

The legacy renderer remains available through SoRenderManager::RenderPipeline::LEGACY_GL, while the new path can be selected with SoRenderManager::RenderPipeline::DRAW_LIST or the COIN_USE_DRAW_LIST environment variable.

Main changes

Modern OpenGL foundations

• Add core-profile OpenGL detection and configuration.
• Add OpenGL debug callback support.
• Add modern GL function loading and related build integration.
• Improve EGL and offscreen-context support.
• Add shader-backed rendering paths where compatibility-mode rendering is unavailable.

Retained rendering infrastructure

• Add SoIRRenderAction, SoDrawList, and retained render commands.
• Record geometry, materials, transforms, raster state, lighting, textures, and picking metadata.
• Add transient geometry allocation with opaque save points for partial rebuilds.
• Keep retained-rendering implementation helpers private.

OpenGL draw-list backend

• Add VAO, VBO, vertex-layout, shader, and geometry-cache management.
• Add opaque, transparent, foreground, selection, and after-main render stages.
• Add retained lighting, wide-line and wide-point rendering, stipple, alpha testing, blending, depth state, and textured commands.
• Add GPU ID-buffer picking and selection/highlight rendering.
• Restore and invalidate shared OpenGL state when switching between rendering paths.

Scene-graph integration

• Add retained traversal support for generated shapes, primitive caches, markers, images, text, lights, annotations, and render-layer groups.
• Preserve pixel-aligned text placement and transparent text traversal.
• Carry explicit shading, two-sided lighting, material, and pipeline state through the retained IR.

SoRenderManager integration

• Add the RenderPipeline and LightingMode APIs.
• Add background and foreground render-layer roots.
• Add scene and foreground invalidation APIs.
• Support partial foreground rebuilds without rebuilding the retained main scene.
• Add callbacks between main-scene and foreground traversal.
• Add an explicit after-main render stage with a depth barrier.
• Safely clear retained selection state and invalidate shared GL state when switching pipelines.

Compatibility

The new renderer is additive. The legacy OpenGL path remains the default unless the draw-list pipeline is explicitly enabled.

The retained path aims to preserve existing Coin rendering semantics, including:

• Render modes and lighting modes.
• Transparency ordering and transparent text.
• Material and vertex colors.
• Depth, blend, alpha-test, line, and point state.
• Background, main-scene, after-main, and foreground ordering.
• Selection and preselection highlighting.

History structure

The commit series is intentionally split into small, dependency-ordered changes:

1. Modern OpenGL and build foundations.
2. Retained IR and traversal infrastructure.
3. Backend resources and render passes.
4. Retained-node coverage.
5. SoRenderManager policy and integration.
6. Invalidation and safe pipeline switching.
7. Layered and after-main rendering.
8. Raster, lighting, text, and compatibility refinements.
9. Manager-level tests.

Testing

Added coverage for:

• Render-manager defaults and policy state.
• Pipeline switching.
• Shared-context state invalidation.
• Device-pixel-ratio state.
• Scene and foreground invalidation.
• After-main callback retention.
• Render-stage ordering and depth barriers.
• Background and hidden-line behavior.

Powers the new FreeCAD renderer at FreeCAD/FreeCAD#31143

[tritao]

This work has finally reached a good enough state where it makes sense to open a PR.

It now passes all visual tests in FreeCAD, though there are still some selection-related improvements I want to make, think I should be able to finish it in the next few days.

@Lgt2x Is this something you are interested in reviewing?

[Lgt2x]

It will take me a while to go through it but yes, I will test & review. Is @oursland in the loop for this?

[tritao]

Superseded by native PR #33, which uses the same integration branch and can participate in the native stacked PR chain.