WebGLRenderer: Prepare shadow maps without rendering during compile().

[hybridherbst]

Related issue: stacked on #34219

Description

When a WebGL node material receives shadows, shader setup needs the shadow map and its depth texture. compile() currently creates these resources by calling shadowMap.render().

Besides rendering actual shadow passes during precompilation (for no reason), this breaks when automatic shadow updates are disabled:

renderer.shadowMap.enabled = true;
renderer.shadowMap.autoUpdate = false;

pointLight.castShadow = true;
mesh.receiveShadow = true;

await renderer.compileAsync( mesh, camera, scene );

Since shadowMap.render() returns before allocating anything, node setup fails with:

TypeError: Cannot read properties of null (reading 'depthTexture')

This change separates shadow-map allocation from rendering, so compile() prepares the required targets without clearing or drawing into them.

I also added a new webgl_tsl_compile_async sample scene (that would fail before this PR).

This contribution is funded by Needle

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	366.61 86.92	366.85 87.01	+238 B +91 B
WebGPU	696.95 192.4	696.95 192.4	+0 B +0 B
WebGPU Nodes	694.94 192.09	694.94 192.09	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	509.99 123.86	510.23 123.94	+238 B +80 B
WebGPU	771.57 207.28	771.57 207.28	+0 B +0 B
WebGPU Nodes	720.53 194.62	720.53 194.62	+0 B +0 B

[github-actions]

🖼️ E2E screenshot tests

✅ All examples render correctly again (run).

[hybridherbst]

@Mugen87 turns out current dev also fails with

renderer.shadowMap.enabled = true;
renderer.shadowMap.autoUpdate = false;

so I think this adjustment to shadow code (to make it reusable and to fix the bug) is justified – let me know what you think!

[Mugen87]

Um, I'm not yet sure it's worth the effort. I'm not supporting a 100% integration of the node materials in WebGLRenderer since that would cause massive additional maintenance effort. WebGLNodesHandler is intended to make the transition to WebGPURenderer easier. So I think it's okay if there are some limitations/restrictions. If the compile() does not work with disabled automatic shadow updates (which is an edge case), we could simply document that before doing a larger refactoring in WebGLShadowMap.