WebGLBackend: Align copyTextureToTexture with WebGLRenderer

[shotamatsuda]

Fixes: #31807

Description

This PR changes the implementation of copyTextureToTexture for WebGLBackend so that it aligns with the one in WebGLRenderer.

It does not handle uninitialized textures the way WebGLRenderer does, because it's out of my hands.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.91 79.11	338.91 79.11	+0 B +0 B
WebGPU	579.78 159.79	581.18 160.03	+1.4 kB +240 B
WebGPU Nodes	578.39 159.55	579.79 159.79	+1.4 kB +239 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.78 113.9	470.78 113.9	+0 B +0 B
WebGPU	649.44 175.54	650.84 175.78	+1.4 kB +236 B
WebGPU Nodes	603.54 164.69	604.94 164.92	+1.4 kB +231 B

[Mugen87]

This PR also fixes the below runtime error that I originally wanted to investigate the next days^^:

Uncaught TypeError: WeakMap key undefined must be an object

It did occur in the first frame in certain examples like webgpu_postprocessing_traa.

[shotamatsuda]

Oh, fancy that I've just been debugging the same error in my TAA (depth texture on my end) 😄