Fix run_on_demand exiting on consequent call

Fixes #3284.

src/platform_impl/macos/app_state.rs

@@ -210,6 +210,10 @@ impl Handler {
         self.exit.store(true, Ordering::Relaxed)
     }
 
+    pub fn clear_exit(&self) {
+        self.exit.store(false, Ordering::Relaxed)
+    }
+
     pub fn exiting(&self) -> bool {
         self.exit.load(Ordering::Relaxed)
     }
@@ -435,6 +439,10 @@ impl AppState {
         HANDLER.exit()
     }
 
+    pub fn clear_exit() {
+        HANDLER.clear_exit()
+    }
+
     pub fn exiting() -> bool {
         HANDLER.exiting()
     }

src/platform_impl/macos/event_loop.rs

@@ -122,6 +122,10 @@ impl<T: 'static> EventLoopWindowTarget<T> {
         AppState::exit()
     }
 
+    pub(crate) fn clear_exit(&self) {
+        AppState::clear_exit()
+    }
+
     pub(crate) fn exiting(&self) -> bool {
         AppState::exiting()
     }