Add missing get_api stubs.

Add hidpi_factor() support for retina displays.
Fix Resize and MouseMoved events to handle retina displays.
Fix inverted y position for MouseMoved events on mac.
Fix initial painting on retina display.

src/x11/window/mod.rs

@@ -658,4 +658,8 @@ impl Window {
             ffi::XFlush(self.x.display);
         }
     }
+
+    pub fn hidpi_factor(&self) -> f32 {
+        1.0
+    }
 }