[WebGPU] Set surfaceConfiguration alphaMode default value to auto (em…

…scripten-core#22006)

As raised in emscripten-core#21939 (comment), the default value for surfaceConfiguration alphaMode
should be "auto", not "opaque".

Relevant Dawn CL: https://dawn-review.googlesource.com/c/dawn/+/190080

system/include/webgpu/webgpu_cpp.h

@@ -1560,7 +1560,7 @@ namespace wgpu {
         TextureUsage usage = TextureUsage::RenderAttachment;
         size_t viewFormatCount = 0;
         TextureFormat const * viewFormats;
-        CompositeAlphaMode alphaMode = CompositeAlphaMode::Opaque;
+        CompositeAlphaMode alphaMode = CompositeAlphaMode::Auto;
         uint32_t width;
         uint32_t height;
         PresentMode presentMode = PresentMode::Fifo;