Fix run_on_demand exiting on consequent call

Fixes #3284.

src/platform_impl/windows/event_loop.rs

@@ -530,6 +530,10 @@ impl<T> EventLoopWindowTarget<T> {
         self.runner_shared.exit_code().is_some()
     }
 
+    pub(crate) fn clear_exit(&self) {
+        self.runner_shared.clear_exit();
+    }
+
     fn exit_code(&self) -> Option<i32> {
         self.runner_shared.exit_code()
     }

src/platform_impl/windows/event_loop/runner.rs

@@ -163,6 +163,10 @@ impl<T> EventLoopRunner<T> {
         self.exit.get()
     }
 
+    pub fn clear_exit(&self) {
+        self.exit.set(None);
+    }
+
     pub fn should_buffer(&self) -> bool {
         let handler = self.event_handler.take();
         let should_buffer = handler.is_none();