KTX2Loader: Fix WebGPU feature detection.

[Mugen87]

Fixed #31690.

Description

As mentioned in #31690 (comment), the feature detection in KTX2Loader for WebGPURenderer is indeed incorrect.

The spec has slightly changed and the PR updates the feature detection to reflect that:

• texture-compression-bc now includes the "S3TC", "RGTC", and "BPTC" formats.
• texture-compression-bptc has been removed (included in texture-compression-bc).
• texture-compression-etc1 has been removed (included in texture-compression-etc2).

[Mugen87]

BC7RGBAUnormSRGB had a wrong value.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.91 79.1	338.91 79.1	+0 B +0 B
WebGPU	578.73 159.37	578.78 159.39	+50 B +20 B
WebGPU Nodes	577.34 159.13	577.38 159.14	+50 B +18 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.7 113.91	470.7 113.91	+0 B +0 B
WebGPU	649.28 175.4	649.33 175.42	+50 B +17 B
WebGPU Nodes	603.38 164.55	603.43 164.57	+50 B +17 B

[Mugen87]

Since ETC2 is backward compatible with ETC1, we can use the same approach like in WebGLRenderer and handle ETC1 as ETC2:

three.js/src/renderers/webgl/WebGLUtils.js

Line 111 in 5011da1

if ( p === RGB_ETC1_Format || p === RGB_ETC2_Format ) return ( transfer === SRGBTransfer ) ? extension.COMPRESSED_SRGB8_ETC2 : extension.COMPRESSED_RGB8_ETC2;