WebGLRenderer: Save/restore textureUnits on render state stack

[SSopin]

Fixed #33207

Description

Fixes nested render calls (e.g. PMREMGenerator inside setProgram)
corrupting the shared textureUnits counter, which caused
GL_INVALID_OPERATION errors on shadow map sampler uniforms.

This contribution is funded by Wearitar

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	359.97 85.53	360.15 85.57	+178 B +41 B
WebGPU	632.13 175.39	632.13 175.39	+0 B +0 B
WebGPU Nodes	630.25 175.1	630.25 175.1	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	492.17 120.11	492.34 120.15	+178 B +38 B
WebGPU	704.56 190.19	704.56 190.19	+0 B +0 B
WebGPU Nodes	653.78 177.6	653.78 177.6	+0 B +0 B

[travisbreaks]

Clean fix for a real bug. Nested render calls (PMREMGenerator, render-to-texture, shadow maps) corrupting the shared textureUnits counter is exactly the kind of issue that manifests as intermittent GL_INVALID_OPERATION errors that are hard to track down.

The save/restore pattern on the render state stack is the correct approach.

One observation:

Fallback behavior

textures.setTextureUnits( currentRenderState.state.textureUnits || 0 );

The `|| 0` fallback silently defaults to 0 if `textureUnits` is undefined (i.e., the render state was created before this change, or through a code path that skips the save). This is safe at runtime, but it masks a potential bug: if a render state somehow lacks `textureUnits`, silently resetting to 0 could cause incorrect texture bindings in the restored context.

Consider whether this should be initialized explicitly in the render state constructor/init instead:

currentRenderState.init( camera );
// textureUnits would already be defined as 0 from init

That way the `|| 0` becomes truly unnecessary rather than a silent safety net.

Minor point, not a blocker. The fix itself is correct and well-scoped.

[SSopin]

Clean fix for a real bug. Nested render calls (PMREMGenerator, render-to-texture, shadow maps) corrupting the shared textureUnits counter is exactly the kind of issue that manifests as intermittent GL_INVALID_OPERATION errors that are hard to track down.

The save/restore pattern on the render state stack is the correct approach.

One observation:

Fallback behavior

textures.setTextureUnits( currentRenderState.state.textureUnits || 0 );

The || 0 fallback silently defaults to 0 if textureUnits is undefined (i.e., the render state was created before this change, or through a code path that skips the save). This is safe at runtime, but it masks a potential bug: if a render state somehow lacks textureUnits, silently resetting to 0 could cause incorrect texture bindings in the restored context.

Consider whether this should be initialized explicitly in the render state constructor/init instead:

currentRenderState.init( camera );
// textureUnits would already be defined as 0 from init

That way the || 0 becomes truly unnecessary rather than a silent safety net.

Minor point, not a blocker. The fix itself is correct and well-scoped.

Fixed.