Implement Windowed Fullscreen

There are two kinds of fullscreen. One where you take over the whole
output the other where you just set the window size to the screen
size and get rid of decorations. The first one already existed,
implement the second which is more common for normal desktop apps.
Use an enum to consolidate all the fullscreen states.

src/lib.rs

@@ -356,6 +356,24 @@ pub enum MouseCursor {
     RowResize,
 }
 
+/// Describes if the Window is in one of the fullscreen modes
+#[derive(Clone, PartialEq)]
+pub enum FullScreenState {
+    None,
+    Windowed,
+    Exclusive(platform::MonitorId),
+}
+
+impl FullScreenState {
+    pub fn get_monitor(&self) -> Option<platform::MonitorId> {
+        if let FullScreenState::Exclusive(ref monitor) = *self {
+            Some(monitor.clone())
+        } else {
+            None
+        }
+    }
+}
+
 /// Describes how winit handles the cursor.
 #[derive(Debug, Copy, Clone, PartialEq)]
 pub enum CursorState {
@@ -392,10 +410,10 @@ pub struct WindowAttributes {
     /// The default is `None`.
     pub max_dimensions: Option<(u32, u32)>,
 
-    /// If `Some`, the window will be in fullscreen mode with the given monitor.
+    /// Whether the window should be set as fullscreen upon creation.
     ///
     /// The default is `None`.
-    pub monitor: Option<platform::MonitorId>,
+    pub fullscreen: FullScreenState,
 
     /// The title of the window in the title bar.
     ///
@@ -435,9 +453,9 @@ impl Default for WindowAttributes {
             dimensions: None,
             min_dimensions: None,
             max_dimensions: None,
-            monitor: None,
             title: "winit window".to_owned(),
             maximized: false,
+            fullscreen: FullScreenState::None,
             visible: true,
             transparent: false,
             decorations: true,