Fix double-sided normals for negative scale meshes

[issam3105]

Objective

Fix incorrect PBR lighting for double-sided meshes with negative-scale transforms.

#8859 fixed culling for single-sided glTF materials by detecting inverted scale in the loader. Double-sided materials still need a shader-side fix, because culling is disabled and raw front_facing is reversed by negative-scale winding.

Solution

Add a winding-corrected front-facing helper in the PBR shader path.

Testing

Tested with Khronos NegativeScaleTest

The bottom-right -1.0 column should match the expected lighting/reflection behavior for double-sided materials.

To validate non-regression for normal/tangent handling, reviewers can also test:

NormalTangentMirrorTest
NormalTangentTest

Showcase

Before

After

[kfc35]

To the more rendering savvy reviewers who would know, should this fix also be put into the array_texture.wgsl files (under assets/shaders and examples/wasm/assets/shaders)? There is code there that also prepare world normals and applies normal mappings, although presumably it’s for examples / showcasing and not used by the engine itself

Beyond that, this code looks good to me and makes sense mathematically

[goodartistscopy]

Nitpicking on the naming: s/mesh_is_front/is_logically_front

[goodartistscopy]

To the more rendering savvy reviewers who would know, should this fix also be put into the array_texture.wgsl files (under assets/shaders and examples/wasm/assets/shaders)? There is code there that also prepare world normals and applies normal mappings, although presumably it’s for examples / showcasing and not used by the engine itself

Indeed, either replicate this or add a comment for properly handling handedness-reversing transformations ?

[goodartistscopy]

Looks sensible apart from a small naming nit