Cocoa: Only create per-monitor display link once

The display link query is not specific to any particular display mode
and so only needs to be done once.

The next step is to replace the display link altogether by querying
IOKit directly, which is what the display link does.

src/cocoa_platform.h

@@ -170,6 +170,7 @@ typedef struct _GLFWmonitorNS
     CGDisplayModeRef    previousMode;
     uint32_t            unitNumber;
     id                  screen;
+    double              fallbackRefreshRate;
 
 } _GLFWmonitorNS;