Fix egui_glow when targeting wasm32-unknown-unknown (#1303)

* Gate winit/glow and epi correctly

* Add check to CI

* Fix epi cfg

.github/workflows/rust.yml

@@ -56,6 +56,20 @@ jobs:
           command: check
           args: -p egui_demo_app --lib --target wasm32-unknown-unknown
 
+  check_wasm_eframe_with_features:
+    name: cargo check wasm eframe
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: 1.56.0
+          override: true
+      - run: rustup target add wasm32-unknown-unknown
+      - name: check
+        run: cargo check -p eframe --lib --no-default-features --features egui_glow,persistence --target wasm32-unknown-unknown
+
   check_web_all_features:
     name: cargo check web --all-features
     runs-on: ubuntu-latest