WebGPU: Support dual-source-blending ?

`dual-source-blending` is a very useful WebGPU extension.

**Use cases:**

- 2D rendering: implement all Porter-Duff blend modes.
- Volume rendering: Tyndall effect etc.

**Potential task**

- Map blend modes: eg: `BLEND_FACTOR_SRC1_COLOR` to `WGPUBlendFactor_Src1`
- WGSL Shader code converter: `enable dual_source_blending;`, `@location(0) @blend_src(1)`