FrontFacingNode: Fix frontFacing if used material.flatShading

[sunag]

Related issue: #30823 (comment)

Description

Fix frontFacing if used material.flatShading.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.08 78.29	336.08 78.29	+0 B +0 B
WebGPU	532.77 148.04	532.74 148.03	-25 B -11 B
WebGPU Nodes	532.24 147.94	532.21 147.93	-25 B -12 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	465.09 112.16	465.09 112.16	+0 B +0 B
WebGPU	604.41 164.01	604.38 164	-25 B -9 B
WebGPU Nodes	559.39 153.46	559.37 153.45	-25 B -9 B

[WestLangley]

@sunag WebGPU is looking better! :-)

There are still console errors for both backends when the geometry has tangents, a normal map is present, and flat shading is true.

If it is not an easy fix, I can file a bug report if you wish.

[Mugen87]

If you share a fiddle, I can take a look!

[sunag]

Hi! Thanks @WestLangley. @Mugen87 I fixed here, I will post this PR soon.