Implement EdgeEffect

Description

Related: #393

Implement an EdgeEffect.

This effect should focus only on rendering object feature edges for the whole scene. Selective object highlighting will be implemented in a different way.

References:

• https://roystan.net/articles/outline-shader/
• https://gamedev.stackexchange.com/questions/68401/how-can-i-draw-outlines-around-3d-models
• https://docs.blender.org/manual/en/2.79/render/blender_render/post_processing/edges.html
• https://github.com/gkjohnson/threejs-sandbox/tree/master/sobel-outline
• https://www.vertexfragment.com/ramblings/unity-postprocessing-sobel-outline/

Tasks

• Create SobelMaterial and SobelPass.
• Create EdgeEffect that uses SobelPass to generate an edge texture.
• Create edge demo under special-effects.
• Add unit test.

Implementation Details

• Use G-Buffer components (depth, normals).
• Edge threshold, color and thickness should be configurable.

Start with related issue #393 and the listed Sobel-outline references to understand the intended rendering approach. The issue names the required entry points—SobelMaterial, SobelPass, EdgeEffect, the special-effects edge demo, and a unit test; done means all four tasks are implemented with configurable threshold, color, and thickness.