Add support for WebGPU dual-source blending

[mvaligursky]

Feature

Add support for dual-source blending (WebGPU dual-source-blending feature + WGSL @blend_src attribute), shipped in Chrome 130.

Dual-source blending lets a fragment shader output two color values (src0/src1) that are both fed into the blend equation, enabling blend factors such as src1 / one-minus-src1. The engine currently does not request the dual-source-blending device feature and has no pipeline/blend-state support for @blend_src.

Benefit

• Subpixel-accurate font/text antialiasing (per-channel coverage).
• Advanced compositing and custom blend operations that a single source color cannot express.

Notes

• Requires requesting the dual-source-blending GPUFeatureName at device creation, exposing a capability flag, and plumbing the second fragment output through blend state.
• Ref: https://developer.chrome.com/blog/new-in-webgpu-130#dual_source_blending