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"))]