Make 'primary_monitor' return 'Option<MonitorHandle>'

Certain platforms like Wayland don't have a concept of
primary Monitor in particular. To indicate that
'primary_monitor' will return 'None' as well as in cases
where the primary monitor can't be detected.

Fixes #1683.

src/platform_impl/ios/event_loop.rs

@@ -13,6 +13,7 @@ use crate::{
     event_loop::{
         ControlFlow, EventLoopClosed, EventLoopWindowTarget as RootEventLoopWindowTarget,
     },
+    monitor::MonitorHandle as RootMonitorHandle,
     platform::ios::Idiom,
 };
 
@@ -56,9 +57,11 @@ impl<T: 'static> EventLoopWindowTarget<T> {
         unsafe { monitor::uiscreens() }
     }
 
-    pub fn primary_monitor(&self) -> MonitorHandle {
+    pub fn primary_monitor(&self) -> Option<RootMonitorHandle> {
         // guaranteed to be on main thread
-        unsafe { monitor::main_uiscreen() }
+        let monitor = unsafe { monitor::main_uiscreen() };
+
+        Some(RootMonitorHandle { inner: monitor })
     }
 }
 

src/platform_impl/ios/window.rs

@@ -276,8 +276,9 @@ impl Inner {
         unsafe { monitor::uiscreens() }
     }
 
-    pub fn primary_monitor(&self) -> MonitorHandle {
-        unsafe { monitor::main_uiscreen() }
+    pub fn primary_monitor(&self) -> Option<RootMonitorHandle> {
+        let monitor = unsafe { monitor::main_uiscreen() };
+        Some(RootMonitorHandle { inner: monitor })
     }
 
     pub fn id(&self) -> WindowId {