Add support for KHR_materials_retroreflection

[mklefrancois]

Summary

Implements KHR_materials_retroreflection, based on the Minimal Retroreflective Microfacet (MRM) model (Portsmouth et al., JCGT 15(1), 2026).

• Substitutes the view direction with its reflection about the normal (V_retro = reflect(-V, N)) before evaluating the metallic and dielectric BRDFs, then linearly blends the result with the regular BRDF by retroreflectionFactor.
• Applied to both the IBL path (getIBLRadianceGGXRetroreflection in ibl.glsl) and the punctual-light path (pbr.frag).
• Adds KHR_materials_retroreflection material extension parsing (retroreflectionFactor, retroreflectionTexture), the MATERIAL_RETROREFLECTION shader define, and a Retroreflection Strength debug output channel.

A companion PR to glTF-Sample-Viewer adds the corresponding "Retroreflection" UI toggle and bumps this submodule pointer, and references glTF-Sample-Viewer#664.

Test plan

• npm run build succeeds with no shader compile errors
• Loaded the glTF-Sample-Assets#285 traffic cone test asset (retroreflective sleeve using retroreflectionFactor: 1.0, masked by retroreflectionTexture) — the retroreflective stripe reads noticeably brighter than the plain reflective stripe when camera and light are closely aligned, matching the extension's reference behavior

[CLAassistant]

All committers have signed the CLA.

[emackey]

For UX3D: The PBR TSG is requesting that you fully check the PBR shader math and check how well it matches the draft spec provided in KhronosGroup/glTF#2610 (because the author is the same between both). Thanks!