Texture: Always generate mipmaps when "generateMipmaps" is true.

[gkjohnson]

Fixed #18861

Description

Change the behavior of Texture mipmap generation to always respect the generateMipmaps flag. Previously mipmaps would only be generated when "minFilter" and "magFilter" required mip maps, as well, but with default support for textureLod in WebGL2 and textureSampleLevel in WebGPU, filtering is no longer the only way to use mipmaps.

This change will mean that mipmaps will be generated for all non-data textures by default; even when users set filtering to use NEAREST. This will potentially result in higher memory usage and a (small?) performance cost even if they're not used.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	690.8 171.12	690.77 171.12	-36 B -3 B
WebGPU	816.43 220.08	816.39 220.05	-44 B -22 B
WebGPU Nodes	815.94 219.95	815.9 219.92	-44 B -22 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	463.45 111.9	463.41 111.89	-34 B -3 B
WebGPU	538.61 145.41	538.57 145.38	-42 B -25 B
WebGPU Nodes	494.73 135.28	494.69 135.26	-42 B -17 B

[Mugen87]

To better reflect the change, we should update example code that uses nearest or linear filtering without requiring mipmaps. E.g.: webgl_materials_texture_filters.

Looking at this specific example, the textures texturePainting, texturePainting2 and textureCanvas2 should now set generateMipmaps to false.

WebGPURenderer must also be updated in this PR. The relevant line is:

three.js/src/renderers/common/Textures.js

Line 340 in 1360b24

return ( texture.isCompressedTexture === true ) || ( ( texture.minFilter !== NearestFilter ) && ( texture.minFilter !== LinearFilter ) );