Make WindowBuilder Send + Sync

Window builder is always accessed by winit on the thread event loop
is on, thus it's safe to mark the data it gets as `Send + Sync`.
Each unsafe object is marked individually as `Send + Sync` instead
of just implementing `Send` and `Sync` for the whole builder.

src/lib.rs

@@ -163,3 +163,18 @@ mod platform_impl;
 pub mod window;
 
 pub mod platform;
+
+/// Wrapper for objects which winit will access on the main thread so they are effectively `Send`
+/// and `Sync`, since they always excute on a single thread.
+///
+/// # Safety
+///
+/// Winit can run only one event loop at the time and the event loop itself is tied to some thread.
+/// The objects could be send across the threads, but once passed to winit, they execute on the
+/// mean thread if the platform demands it. Thus marking such objects as `Send + Sync` is safe.
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub(crate) struct SendSyncWrapper<T>(pub(crate) T);
+
+unsafe impl<T> Send for SendSyncWrapper<T> {}
+unsafe impl<T> Sync for SendSyncWrapper<T> {}