WebGLPrograms: Don't use flat shading with enabled wireframe.

[Mugen87]

Fixed #31238.

Description

The PR makes sure that flat shading is only used when material.wireframe is disabled.

Note that when toggling material.wireframe with enabled flat shading, you must set material.needsUpdate to true so the shader gets updated correctly. That is a small compromise since checking the wireframe property per frame adds additional cost to the renderer and that is potentially not appropriated for this edge case.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	337.4 78.67	337.42 78.68	+18 B +3 B
WebGPU	554.99 153.61	554.99 153.61	+0 B +0 B
WebGPU Nodes	554.34 153.46	554.34 153.46	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	468.59 113.33	468.61 113.33	+18 B +3 B
WebGPU	630.02 170.38	630.02 170.38	+0 B +0 B
WebGPU Nodes	584.87 159.73	584.87 159.73	+0 B +0 B