Add opt-out fragile-send-sync-non-atomic-wasm feature for wgpu (#5098)

Note this will break people depending on eframe or egui-wgpu with
--no-default-features.
I don't know what to do about that to be honest.

* Closes #4914 
* [x] I have followed the instructions in the PR template

---------

Co-authored-by: Andreas Reich <r_andreas2@web.de>

crates/egui-wgpu/src/lib.rs

@@ -173,6 +173,9 @@ impl RenderState {
             dithering,
         );
 
+        // On wasm, depending on feature flags, wgpu objects may or may not implement sync.
+        // It doesn't make sense to switch to Rc for that special usecase, so simply disable the lint.
+        #[allow(clippy::arc_with_non_send_sync)]
         Ok(Self {
             adapter: Arc::new(adapter),
             #[cfg(not(target_arch = "wasm32"))]

crates/egui-wgpu/src/renderer.rs

@@ -7,8 +7,19 @@ use epaint::{emath::NumExt, PaintCallbackInfo, Primitive, Vertex};
 
 use wgpu::util::DeviceExt as _;
 
+// Only implements Send + Sync on wasm32 in order to allow storing wgpu resources on the type map.
+#[cfg(not(all(
+    target_arch = "wasm32",
+    not(feature = "fragile-send-sync-non-atomic-wasm"),
+)))]
 /// You can use this for storage when implementing [`CallbackTrait`].
 pub type CallbackResources = type_map::concurrent::TypeMap;
+#[cfg(all(
+    target_arch = "wasm32",
+    not(feature = "fragile-send-sync-non-atomic-wasm"),
+))]
+/// You can use this for storage when implementing [`CallbackTrait`].
+pub type CallbackResources = type_map::TypeMap;
 
 /// You can use this to do custom [`wgpu`] rendering in an egui app.
 ///
@@ -1017,6 +1028,11 @@ impl ScissorRect {
     }
 }
 
+// Look at the feature flag for an explanation.
+#[cfg(not(all(
+    target_arch = "wasm32",
+    not(feature = "fragile-send-sync-non-atomic-wasm"),
+)))]
 #[test]
 fn renderer_impl_send_sync() {
     fn assert_send_sync<T: Send + Sync>() {}