WebGLBackend: Honor layerUpdates for array textures.

[ahaensler]

Related issue: none, see below

Description

The WebGL fallback always uploads the complete array texture. That's too slow when I only change some layers.

The PR enables partial uploads using texture.layerUpdates.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.63 79.03	338.63 79.03	+0 B +0 B
WebGPU	567.42 156.92	567.73 157.03	+305 B +105 B
WebGPU Nodes	566.03 156.68	566.33 156.78	+305 B +106 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.38 113.78	470.38 113.78	+0 B +0 B
WebGPU	639.16 172.99	641.46 173.51	+2.3 kB +521 B
WebGPU Nodes	593.81 162.25	596.11 162.81	+2.3 kB +553 B

[Mugen87]

Additional info: This code was already present in WebGLRenderer and has now been ported to the WebGL backend as well.