Fix monitor disappearing on OS X with AGS

When automatic graphics switching is activated, the CGDirectDisplayID
for the display changes, casuing the primary GLFWmonitor to disappear.

src/cocoa_platform.h

@@ -90,6 +90,7 @@ typedef struct _GLFWmonitorNS
 {
     CGDirectDisplayID   displayID;
     CGDisplayModeRef    previousMode;
+    uint32_t            unitNumber;
 
 } _GLFWmonitorNS;