Examples: Add DepthSavePass and depth texture tools

[gchoqueux]

Description

Add DepthSavePass, depth texture utilities, and interactive displacement example

This DepthSavePass introduces support for working with depth textures in post-processing workflows and DepthTextureUtils adds a world position picking;

This position picking method based on the depth texture takes into account map displacement. It can also be more performant than raycasting, especially when the mesh has a high number of faces, as it avoids the computational cost of geometric intersection tests.

🧱 New Post-Processing Pass

• DepthSavePass
  A render pass that saves the scene’s depth buffer into a Texture.
  Useful for accessing per-pixel depth values using renderer.readRenderTargetPixels() or for use in subsequent post-processing effects.

🛠️ Depth Utilities

• DepthTextureUtils
  A small utility module for working with depth data:
  • pickWorldPosition() — converts screen-space mouse coordinates into a 3D world position using the depth texture.

🧪 Example

• webgl_postprocessing_interactive_displacement.html
  A live demo showcasing:
  • Animated displacement mapping.
  • Real-time mouse interaction using depth-based position picking.

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These additions extend Three.js’s post-processing capabilities and simplify depth buffer access for custom spatial effects.

⚠️ Note: On some Windows configurations, depth values read from the buffer may appear incorrect.