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.

docs/news.dox

@@ -52,6 +52,12 @@ add_subdirectory(path/to/glfw)
 @endcode
 
 
+@subsubsection corevideo_34 CoreVideo dependency has been removed
+
+GLFW no longer depends on the CoreVideo framework on macOS and it no longer
+needs to be specified during compilation or linking.
+
+
 @subsection deprecations_34 Deprecations in version 3.4
 
 @subsection removals_34 Removals in 3.4