WebGLRenderer: Use native depth texture for VSM shadow maps

[mrdoob]

Related issue: #32303

Description

• Refactors VSM shadow mapping to use native depth textures instead of IdentityDepthPacking
• Removes unused IdentityDepthPacking constant and shader code
• Ports the max(0.0, ...) fix from WebGPU to prevent NaN artifacts (WebGPURenderer: Fix VSM shadow artifacts. #32327)

(Made using Claude Code with Opus 4.5)

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.26 83.06	350.39 83.06	+123 B +1 B
WebGPU	614.41 170.62	614.38 170.61	-21 B -7 B
WebGPU Nodes	613.01 170.35	612.99 170.35	-21 B -4 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	482.25 117.84	482.43 117.86	+175 B +21 B
WebGPU	685.72 186.38	685.72 186.38	+0 B +0 B
WebGPU Nodes	635.56 173.57	635.56 173.57	+0 B +0 B