bug#38163: GDK does not return the monitor's refresh rate

[Alexandros Theodotou <alex@zrythm.org>]

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I have been developing and testing my GTK app (Zrythm) on many distros
and Guix is the only case that the monitor's refresh rate is returned
as 0.

The API I am using is this:
https://developer.gnome.org/gdk3/stable/GdkMonitor.html#gdk-monitor-get-refresh-rate

I was developing/testing on Parabola GNU/Linux-libre before and it was
giving me a correct rate of 60 Hz.

Other distros I have tested this API before on:
Various ubuntu versions, debian 9 and 10, manjaro (arch), various
fedora versions, plus a few others that users tested on.

I suspect GDK is trying to access something that guix doesn't make
available to it, but I don't have much knowledge of the internals of
GDK or Guix to debug this myself.

I will post a package from my guix channel that you can test this on
soon.

To call the API, one basically calls the following, after including
<gtk/gtk.h>, so any GTK program will do.
```
GdkDisplay * display = gdk_display_get_default ();
GdkMonitor * monitor = gdk_display_get_primary_monitor (display);
int refresh_rate = gdk_monitor_get_refresh_rate (monitor);
```

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