ui/console: fix crash when using gl context with non-gl listeners

The commit 7cc712e9 ("ui: dispatch GL events to all listener")
mechanically replaced the dpy_gl calls with a dispatch loop, using the
same pre-conditions. However, it didn't take into account that all
listeners do not have to implement the GL callbacks.

Add the missing pre-conditions before calling the callbacks.

Fix crash when running a GL-enabled VM with "-device virtio-gpu-gl-pci
-display egl-headless -vnc :0".

Fixes: 7cc712e9 ("ui: dispatch GL events to all listener")
Reported-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220214201337.1814787-2-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>