When running QEMU's GTK UI without EGL or OGL, the
gd_monitor_update_interval function gets executed and the display refresh
rate gets updated accordingly. However, when using EGL or just regular
OGL, the function never gets executed.

Which is why I decided that the function should be in gd_egl_refresh
where the display output gets updated, in the same vain as how it's done
for normal GTK UIs (aka. those without EGL) - in it's display refresh
function.

Since the gd_monitor_update_interval function now is exposed, we are
going to use it to update the refresh rate.

Signed-off-by: Nikola Pavlica <pavlica.nikola@gmail.com>
Message-Id: <20210114140153.301473-3-pavlica.nikola@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>