Add WindowBuilder::transparent

This is required to help hardware accelerated libraries like glutin
that accept WindowBuilder instead of RawWindowHandle, since the api
to access builder properties directly was removed.

Follow up to 44288f6.

src/window.rs

@@ -278,6 +278,12 @@ impl WindowBuilder {
         self
     }
 
+    /// Get whether the window will support transparency.
+    #[inline]
+    pub fn transparent(&self) -> bool {
+        self.window.transparent
+    }
+
     /// Sets whether the window should have a border, a title bar, etc.
     ///
     /// The default is `true`.