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/Cargo.toml

@@ -31,7 +31,7 @@ all-features = true
 rustdoc-args = ["--generate-link-to-definition"]
 
 [features]
-default = []
+default = ["fragile-send-sync-non-atomic-wasm"]
 
 ## Enable profiling with the [`puffin`](https://docs.rs/puffin) crate.
 puffin = ["dep:puffin"]
@@ -45,6 +45,12 @@ wayland = ["winit?/wayland"]
 ## Enables x11 support for winit.
 x11 = ["winit?/x11"]
 
+## Make the renderer `Sync` on wasm, exploiting that by default wasm isn't multithreaded.
+## It may make code easier, expecially when targeting both native and web.
+## On native most wgpu objects are send and sync, on the web they are not (by nature of the WebGPU specification).
+## This is not supported in [multithreaded WASM](https://gpuweb.github.io/gpuweb/explainer/#multithreading-transfer).
+## Thus that usage is guarded against with compiler errors in wgpu.
+fragile-send-sync-non-atomic-wasm = ["wgpu/fragile-send-sync-non-atomic-wasm"]
 
 [dependencies]
 egui = { workspace = true, default-features = false }