TSL: Fix .blur() on Framebuffer

[RenaudRohlinger]

Related issue: #30722

Description
blur() was broken on TextureNode with Framebuffer value such as ViewportSharedTextureNode. This PR fixes the issue.

This contribution is funded by Renaud Rohlinger @ Utsubo

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.36 78.34	336.36 78.34	+0 B +0 B
WebGPU	533.07 148.15	533.1 148.16	+24 B +7 B
WebGPU Nodes	532.54 148.05	532.56 148.06	+24 B +8 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.37 112.21	465.37 112.21	+0 B +0 B
WebGPU	604.68 164.12	604.71 164.13	+24 B +10 B
WebGPU Nodes	559.67 153.56	559.7 153.56	+24 B +5 B