Make 'current_monitor' return 'Option<MonitorHandle>'

On certain platforms window couldn't be on any monitor
resulting in failures of 'current_monitor' function.

Such issue was happening on Wayland, since the window
isn't on any monitor, unless the user has drawn something into it.

Returning 'Option<MonitorHandle>' will give an ability to
handle such situations gracefully by properly indicating that
there's no current monitor.

Fixes #793.

src/platform_impl/macos/window.rs

@@ -972,7 +972,8 @@ impl UnownedWindow {
     }
 
     #[inline]
-    pub fn current_monitor(&self) -> RootMonitorHandle {
+    // Allow directly accessing the current monitor internally without unwrapping.
+    pub(crate) fn current_monitor_inner(&self) -> RootMonitorHandle {
         unsafe {
             let screen: id = msg_send![*self.ns_window, screen];
             let desc = NSScreen::deviceDescription(screen);
@@ -985,6 +986,11 @@ impl UnownedWindow {
         }
     }
 
+    #[inline]
+    pub fn current_monitor(&self) -> Option<RootMonitorHandle> {
+        Some(self.current_monitor_inner())
+    }
+
     #[inline]
     pub fn available_monitors(&self) -> VecDeque<MonitorHandle> {
         monitor::available_monitors()

src/platform_impl/macos/window_delegate.rs

@@ -450,7 +450,8 @@ extern "C" fn window_will_enter_fullscreen(this: &Object, _: Sel, _: id) {
                 // Otherwise, we must've reached fullscreen by the user clicking
                 // on the green fullscreen button. Update state!
                 None => {
-                    shared_state.fullscreen = Some(Fullscreen::Borderless(window.current_monitor()))
+                    let current_monitor = window.current_monitor_inner();
+                    shared_state.fullscreen = Some(Fullscreen::Borderless(current_monitor))
                 }
             }
             shared_state.in_fullscreen_transition = true;