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/osx/headless.rs

@@ -97,6 +97,10 @@ impl HeadlessContext {
         };
         symbol as *const ()
     }
+
+    pub fn get_api(&self) -> ::Api {
+        ::Api::OpenGl
+    }
 }
 
 unsafe impl Send for HeadlessContext {}