SFML's render window only supports X11 under Linux, so SLADE crashes if GDK
chooses the Wayland backend. This patch tells GDK to prefer the X11 backend.
SLADE 3.3.0 will apparently not use the render window.

diff --git a/src/Application/SLADEWxApp.cpp b/src/Application/SLADEWxApp.cpp
index 3dadf241..de01e340 100644
--- a/src/Application/SLADEWxApp.cpp
+++ b/src/Application/SLADEWxApp.cpp
@@ -52,6 +52,16 @@
 using namespace slade;


+#ifdef __WXGTK__
+#include <gdk/gdk.h>
+
+struct PreferX11 {
+	PreferX11() { gdk_set_allowed_backends("x11,*"); }
+};
+static PreferX11 preferx11;
+#endif
+
+
 // -----------------------------------------------------------------------------
 //
 // Variables