ui/gtk: Only try to initialize EGL/X11 if GtkGlArea failed

ui/gtk: Only try to initialize EGL/X11 if GtkGlArea failed

The commit referenced below changed the logic by causing the gtk-egl
backend to be initialized regardless of whether GtkGlArea initialization
succeeded. This causes eglInitialize to crash in Wayland systems without
XWayland.

This patch restores the previous logic.

Fixes: 4c70280592f5 ("ui/gtk: use GtkGlArea on wayland only")
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Message-id: 20180507134237.14996-1-tomeu.vizoso@collabora.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>