Normal: Don't flip normals when using flat shading.

[Mugen87]

Related issue: https://discourse.threejs.org/t/webgpu-flatshading-doubleside-broken/80198

Description

The PR ensures normals are not flipped when using flat shading.

[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	532.64 148.02	532.77 148.04	+127 B +26 B
WebGPU Nodes	532.11 147.91	532.24 147.94	+127 B +26 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	604.28 163.99	604.41 164.01	+127 B +19 B
WebGPU Nodes	559.32 153.43	559.39 153.46	+76 B +22 B

[WestLangley]

If the front side is extruded by a normal map, then the back side should be indented. With this PR, both sides are extruded when flat shading is true.