Make egui_wgpu::RenderPass Send and Sync

[leudz]

I didn't open an issue as this is a tiny change.

I believe RenderPass was supposed to be Send + Sync since the callback has the bounds.
This PR changes the flavor of TypeMap used and make RenderPass Send + Sync.

[emilk]

Good catch! We can also add this to ensure this in the future:

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

[leudz]

Thanks for merging it so quickly!