[webgpu] Support WGPUCompositeAlphaMode_Premultiplied

[SergeyLebedkin]

Browsers already supports Premultiplied alpha mode in surface configuration if use JS way to create webgpu context handles.
But emsdk still have it as TODO option.
Please, make it works.

WGPUSurfaceConfiguration surfaceConfiguration {
    .device = device,
    .format = preferredFormat,
    .usage = WGPUTextureUsage_RenderAttachment,
    .alphaMode = WGPUCompositeAlphaMode_Premultiplied, // <-- does not supported
    .width = w,
    .height = h,
    .presentMode = WGPUPresentMode_Fifo,
};
wgpuSurfaceConfigure(surface, &surfaceConfiguration);

[kainino0x]

Ah... we left this as TODO in #21939 (comment) but forgot to follow up. Thanks for reporting!

I've started trying to fix this.