ui/gtk: -display gtk requires PIXMAN

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>

meson.build

@@ -1531,7 +1531,11 @@ gtkx11 = not_found
 vte = not_found
 have_gtk_clipboard = get_option('gtk_clipboard').enabled()
 
-if not get_option('gtk').auto() or have_system
+if get_option('gtk') \
+             .disable_auto_if(not have_system) \
+             .require(pixman.found(),
+                      error_message: 'cannot enable GTK if pixman is not available') \
+             .allowed()
   gtk = dependency('gtk+-3.0', version: '>=3.22.0',
                    method: 'pkg-config',
                    required: get_option('gtk'))