Fix raytracing material texture selection

[anirul]

Summary

• Preserve per-source opaque/transmissive material textures in raytracing by building shared texture atlases and remapping source UVs into the atlas tiles.
• Carry atlas UV bounds through OpenGL and Vulkan aggregate/skinning paths so static and GPU-updated geometry samples the correct material tile.
• Add a multi-material raytracing sample scene and OpenGL/Vulkan regressions for multiple opaque glTF materials.
• Mark OpenGL-dependent CTest cases, including JSON tests that construct OpenGL-backed objects, and exclude them from GitHub-hosted Windows CI while aligning Linux/Windows PR triggers.

Tests

• git diff --check
• cmake --preset windows
• ctest --test-dir build\windows -C Debug -N -L requires-opengl
• ctest --test-dir build\windows -C Debug -N -LE requires-opengl
• ctest --test-dir build\windows -C Debug --output-on-failure -LE requires-opengl (82/82 passed)
• FrameOpenGLTest.exe --gtest_filter="OpenGLRayTracingLevelTest.ImportsGltfGlassMaterialFromIco:OpenGLRayTracingLevelTest.ImportsGltfOpaqueSpecularFromPlate:OpenGLRayTracingLevelTest.BuildsOpaqueAtlasForMultipleOpaqueGltfMaterials"
• FrameVulkanTest.exe --gtest_filter="VulkanRayTracingDualParseTest.ImportsGltfGlassMaterialFromIco:VulkanRayTracingDualParseTest.ImportsGltfOpaqueSpecularFromPlate:VulkanRayTracingDualParseTest.BuildsOpaqueAtlasForMultipleOpaqueGltfMaterials"

Notes

• Windows CI intentionally excludes tests labeled requires-opengl; Linux keeps OpenGL coverage through xvfb/Mesa.
• The new asset/json/raytracing_multi_material.json scene was JSON-validated but not manually launched.