TSL: viewportTexture() cache FramebufferTexture according to RenderTarget

[sunag]

Fixes #31294 (comment)

Description

Cache FramebufferTexture according to RenderTarget, preventing the same framebuffer texture from being used in different rendering contexts.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	337.69 78.76	337.69 78.76	+0 B +0 B
WebGPU	557.35 154.34	557.75 154.43	+402 B +99 B
WebGPU Nodes	556.27 154.12	556.67 154.22	+402 B +99 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	468.91 113.45	468.91 113.45	+0 B +0 B
WebGPU	632.98 171.34	633.38 171.45	+401 B +111 B
WebGPU Nodes	588.11 160.69	588.51 160.8	+401 B +108 B

[Mugen87]

The root cause for the performance regression (#31565) is this clone.

Viewport texture nodes might be cloned implicitly in TSL e.g. when using sample(). Because each instance of ViewportTextureNode has its own texture cache, we end up with cloned framebuffer textures for each cloned viewport texture node. That was not the case with the previous implementation since the framebuffer texture was unique and shared across all cloned viewport texture nodes.

Because the texture value is the reference for the update map in NodeFrame, we end up with many redundant updateBefore() calls which all call copyFramebufferToTexture(). These calls slow down the renderer significantly.

An easy fix is to share the texture cache in cloned instance. Simply adding this line in clone() fixes the regression:

viewportTextureNode._textures = this._textures;

However, this does not fix the use case when you manually create multiple instance of ViewportTextureNode and share the same framebuffer texture:

const tex1 = viewportTexture( uvNode, levelNode, framebufferTexture ); 
const tex2 = viewportTexture( uvNode, levelNode, framebufferTexture ); 
const tex3 = viewportTexture( uvNode, levelNode, framebufferTexture ); 

[sunag]

Hmm... Thank you for finding the root cause. I think I made a mistake when I put the _textures property as a class property, it should be outside.

I'll check it out more calmly and create a PR