Fix egui-wgpu build on wasm32-unknown-emscripten (#8454)

Fix a build issue in the `egui-wgpu` crate when building for the
`wasm32-unknown-emscripten` target. WGPU does not support the WebGPU
backend on that target.

crates/egui-wgpu/src/setup.rs

@@ -101,7 +101,10 @@ impl WgpuSetup {
                 let mut backends = create_new.instance_descriptor.backends;
 
                 // Don't try WebGPU if we're not in a secure context.
-                #[cfg(target_arch = "wasm32")]
+                // Emscripten is excluded: wgpu gates both its `webgpu` backend and
+                // its `web_sys` re-export on `not(Emscripten)` (see the cfg aliases
+                // in `wgpu/build.rs`).
+                #[cfg(all(target_arch = "wasm32", not(target_os = "emscripten")))]
                 if backends.contains(wgpu::Backends::BROWSER_WEBGPU) {
                     let is_secure_context =
                         wgpu::web_sys::window().is_some_and(|w| w.is_secure_context());