GTK: clip any editors instead of resizing GraphicsWindow.

Such resizing may well get us past OpenGL's maximum texture size
and break rendering.

src/platform/gtkmain.cpp

@@ -360,6 +360,8 @@ private:
 class EditorOverlay : public Gtk::Fixed {
 public:
     EditorOverlay(Gtk::Widget &underlay) : _underlay(underlay) {
+        set_size_request(0, 0);
+
         add(_underlay);
 
         _entry.set_no_show_all(true);