Add missing CGDisplayCreateImage() function.

Author: daniel-abramov

src/display.rs

@@ -76,6 +76,7 @@ extern {
     pub fn CGDisplayPixelsHigh(display: CGDirectDisplayID) -> libc::size_t;
     pub fn CGDisplayPixelsWide(display: CGDirectDisplayID) -> libc::size_t;
     pub fn CGDisplayBounds(display: CGDirectDisplayID) -> CGRect;
+    pub fn CGDisplayCreateImage(display: CGDirectDisplayID) -> CGImageRef;
 
     // mouse stuff
     pub fn CGDisplayHideCursor(display: CGDirectDisplayID) -> CGError;