WebGPU: Support dual-source-blending ?

[WangHoi]

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)