invariance is maintained by top-level EventLoopWindowTarget<T>

this field is transitional and her to keep API compatibility only.
the correct variance and such is already ensured by the top-level
`EventLoopWindowTarget`, just use `PhantomData<T>` here.

src/event_loop.rs

@@ -50,7 +50,7 @@ pub struct EventLoop<T: 'static> {
 /// `&EventLoop`.
 pub struct EventLoopWindowTarget<T: 'static> {
     pub(crate) p: platform_impl::EventLoopWindowTarget<T>,
-    pub(crate) _marker: PhantomData<*mut ()>, // Not Send nor Sync
+    pub(crate) _marker: PhantomData<*mut T>, // Not Send nor Sync + invariant over T
 }
 
 /// Object that allows building the event loop.

src/platform_impl/windows/event_loop.rs

@@ -182,10 +182,7 @@ pub struct EventLoopWindowTarget<T: 'static> {
     thread_id: u32,
     thread_msg_target: HWND,
     pub(crate) runner_shared: EventLoopRunnerShared<UserEventPlaceholder>,
-    // TODO
-    // eventually should be removed after all the backends refactored,
-    // but for now should this be invariant or contra-variant to T?
-    _marker: PhantomData<*mut T>,
+    _marker: PhantomData<T>,
 }
 
 impl<T: 'static> EventLoop<T> {