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/win32/mod.rs

@@ -289,6 +289,10 @@ impl Window {
     pub fn set_cursor(&self, cursor: MouseCursor) {
         unimplemented!()
     }
+
+    pub fn hidpi_factor(&self) -> f32 {
+        1.0
+    }
 }
 
 #[unsafe_destructor]