Regression in 0.22 - egui::Ui is no longer Send + Sync

[idanarye]

With egui 0.21 this code would build

fn foo<T: Sync>() {}

fn main() {
    foo::<egui::Ui>();
}

But when I run it with egui 0.22.0 (or with the current master) I get the following error:

error[E0277]: `(dyn for<'a> Fn(usize, &'a egui::Style) -> Option<Color32> + 'static)` cannot be shared between threads safely
 --> src/main.rs:4:11
  |
4 |     foo::<egui::Ui>();
  |           ^^^^^^^^ `(dyn for<'a> Fn(usize, &'a egui::Style) -> Option<Color32> + 'static)` cannot be shared between threads safely
  |
  = help: the trait `Sync` is not implemented for `(dyn for<'a> Fn(usize, &'a egui::Style) -> Option<Color32> + 'static)`
  = note: required for `Unique<(dyn for<'a> Fn(usize, &'a egui::Style) -> Option<Color32> + 'static)>` to implement `Sync`
  = note: required because it appears within the type `Box<dyn Fn(usize, &Style) -> Option<Color32>>`
  = note: required because it appears within the type `Option<Box<dyn Fn(usize, &Style) -> Option<Color32>>>`
  = note: required because it appears within the type `GridLayout`
  = note: required because it appears within the type `Option<GridLayout>`
  = note: required because it appears within the type `Placer`
  = note: required because it appears within the type `Ui`
note: required by a bound in `foo`
 --> src/main.rs:1:11
  |
1 | fn foo<T: Sync>() {}
  |           ^^^^ required by this bound in `foo`

Having a Send + Sync handle to the UI is useful for some workflow (or maybe just for my specific workflow...), and I think a function that picks a color shouldn't be the thing to break it.

I can make a PR to restrict that Fn to be Send + Sync.

[emilk]

Feel free to open a PR to fix this, and to add a check against future regressions:

#[test]
fn ui_impl_send_sync() {
    fn assert_send_sync<T: Send + Sync>() {}
    assert_send_sync::<Ui>();
}