Add Force Double Sided toggle to Cesium3DTileset

[triktron]

Description

Adds an opt-in Force Double Sided boolean to Cesium3DTileset. When enabled, every per-primitive material has _DoubleSidedEnable = 1 and _Cull / _CullMode / _BUILTIN_CullMode = Off, applied after the per-primitive glTF material values are written.

This makes it possible to assign a custom Opaque Material with Render Face = Both (URP) or its built-in / HDRP equivalent and have that setting actually stick. Currently, the cull state on every cloned material is overwritten on each tile load from gltfMaterial.doubleSided (see #491 for why that override exists), so user-supplied materials with double-sided rendering silently revert to back-face culling.

Default is false, so the existing glTF-spec-driven behavior introduced in #491 is unchanged for anyone who doesn't opt in.

Where the override is applied (native): outside the existing if (pMaterial) { setGltfMaterialParameterValues(...); } block in UnityPrepareRendererResources.cpp, so it runs after the glTF write and also covers primitives that have no glTF material attached.

How the C# property reaches native code: standard Reinterop wiring — _forceDoubleSided field + forceDoubleSided property on Cesium3DTileset, an exposure line in ConfigureReinterop.cs, and tilesetComponent.forceDoubleSided() at the native call site. No header changes — Reinterop generates the binding.

Issue number or link

No tracking issue — small additive feature. Happy to open one if preferred.

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have done a full self-review of my code.
• I have updated the documentation as necessary.

Testing plan

Manual, in both URP and Built-in (have not yet verified HDRP, where the original #491 had quirks).

1. Fresh scene, Cesium3DTileset with Opaque Material = None, Force Double Sided = false — default behavior preserved:
   • Tiles whose glTF has doubleSided = false are back-face culled.
   • Tiles whose glTF has doubleSided = true render from both sides.
2. Same scene, flip Force Double Sided = true — every tile renders both sides. Runtime per-primitive material has _Cull == 0, _CullMode == 0, _DoubleSidedEnable == 1.
3. Duplicate CesiumDefaultTilesetMaterial, set Render Face = Back so _Cull = 2, assign to Opaque Material, toggle Force Double Sided = true — Cesium still renders double-sided (verifies the toggle wins over the source material as well).
4. Same custom material, Force Double Sided = false — cull state matches the glTF (original behavior).
5. Inspector: the new "Force Double Sided" entry appears in the Render section between "Generate Smooth Normals" and "Ignore KHR_materials_unlit", with the tooltip on hover.

[triktron]

thinking about the problem, perhaps its better if we don't force double-sided on, but instead add a toggle that forced the material properties over the gltf. I'll modify the code later this week.