Cocoa: Replace display link with IOKit query

This removes the final dependency on CoreVideo, using a display link to
get the refresh rate of monitors where Core Graphics report a refresh
rate of zero.  Instead we now query the I/O registry directly, similarly
to what the display link does at creation.

Thanks to @OneSadCookie for pointers to this solution.

src/cocoa_platform.h

@@ -28,8 +28,6 @@
 #include <dlfcn.h>
 
 #include <Carbon/Carbon.h>
-#include <CoreVideo/CVBase.h>
-#include <CoreVideo/CVDisplayLink.h>
 
 // NOTE: All of NSGL was deprecated in the 10.14 SDK
 //       This disables the pointless warnings for every symbol we use