WebGL: implement advanced blend modes via fragment shaders

[obiot]

Summary

The following CSS blend modes are only supported by the Canvas renderer and fall back to "normal" in WebGL:

• overlay
• color-dodge
• color-burn
• hard-light
• soft-light
• difference
• exclusion

These modes require per-pixel conditional math that cannot be expressed with gl.blendFunc/gl.blendEquation. The only way to implement them in WebGL is via custom fragment shaders.

Current state

• PR Expand blend mode support with darken/lighten in WebGL2 #1317 added darken/lighten in WebGL2 via gl.MIN/gl.MAX
• setBlendMode returns the actually applied mode, so callers can detect unsupported modes
• The Blend Modes example shows green/red indicators for supported/unsupported modes

Implementation approach

Each advanced blend mode would need:

1. A custom fragment shader with the blend formula (e.g. overlay: base < 0.5 ? 2*base*blend : 1-2*(1-base)*(1-blend))
2. A two-pass render: first pass renders to a texture, second pass blends using the shader
3. Integration with the existing setBlendMode API

Reference

• W3C Compositing and Blending spec — canonical blend formulas