TSL: Fix performance of viewportTexture()

[sunag]

Fixes #31565
Related issue: #31343 (comment)

Description

Fix cache and reference of viewportTexture().

• TexturesNode: Introduce getBase()

[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	567.91 157.09	568.16 157.13	+247 B +46 B
WebGPU Nodes	566.52 156.84	566.77 156.89	+247 B +46 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	641.65 173.56	641.87 173.61	+214 B +41 B
WebGPU Nodes	596.3 162.86	596.51 162.91	+214 B +51 B

[Mugen87]

While doing some performance tests I have noticed a minor performance regression in webgpu_backdrop_water introduced by this PR. When testing in Firefox/WebGL, the framerate goes down from 29 to 19 FPS. With Chrome/WebGPU, there is no such regression.

For comparison:

https://rawcdn.githack.com/mrdoob/three.js/950dbb506725477c3300d5bdde9adbef3bde4d8a/examples/webgpu_backdrop_water.html (the URL uses the first build containing the changes of this PR)
https://threejs.org/examples/webgpu_backdrop_water