Gui: prepare 3D rendering and visual test infrastructure for new renderer

[tritao]

This PR reworks FreeCAD’s current Coin-based renderer and visual test suite around a cleaner retained-mode foundation. Scene construction, screen-space geometry, mesh render data, framebuffer capture, and teardown are made more explicit, deterministic, and reusable while preserving the existing rendering path.

The result gives the follow-up PR a smaller and clearer integration surface for adding the new renderer, with less duplicated scene preparation and stronger visual-test contracts.

Viewer and framebuffer capture

• Separates viewer-owned presentation from the GL render-action scene path.
• Adds renderToImage() for controlled raster capture.
• Keeps grabFramebuffer() for live viewport capture.
• Makes viewer lighting optional during controlled captures.
• Fixes QImage ownership transfer in the Python wrapper.
• Adds explicit framebuffer behavior contracts.

Retained scene-graph infrastructure

• Introduces SoFCScreenSpaceGroup for shared screen-space scene preparation.

• Modernizes retained geometry for:

  • axis cross and NaviCube presentation
  • labels and datum labels
  • spline control points
  • polygon overlays
  • color bars and legends

• Extracts TranslateManip.

• Removes the unused SoTextLabel node.

• Makes background, label, annotation, placement-indicator, and NaviCube rendering more deterministic.

• Separates bounding-box preparation and screen-space geometry state from rendering state.

Visual test infrastructure

• Switches Coin snapshots to capture through the live viewer.
• Centralizes snapshot fixtures, camera policies, framing, and baseline configuration.
• Makes scene construction deterministic for screen-space and overlay nodes.
• Refreshes the Coin baselines for the stabilized capture path.
• Adds coverage for core overlay, transparency, selection, material, label, and mesh-rendering behavior.
• Ensures visual sources can be built with the required material resources available.

Teardown and lifecycle handling

• Drains deferred GUI deletions before Python shutdown.
• Makes viewer and scene teardown more deterministic.
• Ensures retained mesh geometry is invalidated and refreshed when source data changes.

Scope

This PR establishes the retained scene and visual-testing infrastructure needed by the next renderer integration. It does not add the new renderer or bump Coin.

This work is part of the broader rendering and selection modernization effort tracked in FreeCAD/FPA-grant-proposals#35.

[tritao]

Did one more audit on this and found some minor issues worth fixing, sent another push with these:

• Fixed the asymmetric negative ticks in zero-based color scales and added coverage for the complete sequence.
• Made the snapshot size explicit so the requested 512×512 dimensions are always honored.
• Separated the So3DAnnotation fixture transforms so the plate, box, and marker no longer affect each other, then refreshed the baseline.

[pierreporte]

What should I test?

[Lgt2x]

@pierreporte see above #31187 (review) :)

[pierreporte]

I haven’t seen any visual problem with things listed in #31187 (review).

Quite a lot of coin messages in the terminale that do not appear in the weekly:

• Coin warning in SoDB::enableRealTimeSensor(): realtime sensor already on (right at startup)
• Coin warning in SbRotation::setValue(): Quarternion has zero length => undefined rotation. (a lot of those)
• Coin error in SoGroup::removeChild(): tried to remove non-existent child 0x6510e893e4c0 (Separator) (there are a ton of these, with different addresses)

[tritao]

Thanks for testing.

Those are fixed by #31112

[maxwxyz]

Noticed no issues

[Lgt2x]

Good to go then! Individual commits make sense, will not squash on merge