ShadowFilterNode: Fix TSL warning in VSMShadowFilter.

[Mugen87]

Related issue: #32209 (comment)

Description

The PR fixes a TSL warning that has been introduces last release when optimize and refactoring VSM. It's this warning.

THREE.TSL: Return statement used in an inline 'Fn()'. Define a layout struct to allow return values.

In general, it's not supported to use JavaScript return statements inside If() TSL statements.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	357.49 84.84	357.49 84.84	+0 B +0 B
WebGPU	622.14 172.83	622.16 172.83	+27 B +1 B
WebGPU Nodes	620.74 172.59	620.77 172.59	+27 B +1 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	490.27 119.86	490.27 119.86	+0 B +0 B
WebGPU	693.11 188.11	693.14 188.11	+27 B +5 B
WebGPU Nodes	642.5 175.21	642.52 175.21	+27 B +4 B