Bring back 3d functionality for copyTextureToTexture

[zonkypop]

Description

In the WebGLBackend, copyTextureToTexture has only implemented copying for 2D textures. This PR cut and pastes some of the code from the main GL renderer to bring back some functionality.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.65 78.39	336.65 78.39	+0 B +0 B
WebGPU	522.37 144.98	522.75 145.11	+385 B +133 B
WebGPU Nodes	521.83 144.88	522.22 145.01	+385 B +133 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.7 112.26	465.7 112.26	+0 B +0 B
WebGPU	592.8 160.61	593.18 160.74	+385 B +130 B
WebGPU Nodes	547.93 150.06	548.31 150.19	+385 B +131 B