[vulkan linux] Fix native_platform_handle with gtk

Use gtk connection as handle and use it when creating surface.

src/xenia/ui/window_gtk.h

@@ -15,6 +15,7 @@
 
 #include <gdk/gdkx.h>
 #include <gtk/gtk.h>
+#include <xcb/xcb.h>
 
 #include "xenia/base/platform_linux.h"
 #include "xenia/ui/menu_item.h"
@@ -31,7 +32,7 @@ class GTKWindow : public Window {
   ~GTKWindow() override;
 
   NativePlatformHandle native_platform_handle() const override {
-    return nullptr;
+    return connection_;
   }
   NativeWindowHandle native_handle() const override { return window_; }
 
@@ -74,6 +75,7 @@ class GTKWindow : public Window {
  private:
   void Create();
   GtkWidget* window_;
+  xcb_connection_t* connection_;
 
   friend void gtk_event_handler_(GtkWidget*, GdkEvent*, gpointer);
   bool HandleMouse(GdkEventAny* event);