[client] wayland: move libdecor and xdg into seperate backends

This allows us to build with libdecor enabled as the selection to use it
is decided upon at runtime if the compositor `gnome-shell` is detected.
If the libdecor development headers are installed, by default it will
now be compiled in unless overridden by the user at compile time.

client/displayservers/Wayland/interface/desktop.h

@@ -0,0 +1,64 @@
+/**
+ * Looking Glass
+ * Copyright © 2017-2023 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#ifndef _H_WAYLAND_DESKTOP_H_
+#define _H_WAYLAND_DESKTOP_H_
+
+#include <stdbool.h>
+#include <wayland-client.h>
+
+typedef struct WL_DesktopOps
+{
+  // the friendly name
+  const char * name;
+
+  // the compositor process name to match
+  const char * compositor;
+
+  bool (*shellInit)(
+    struct wl_display * display, struct wl_surface * surface,
+    const char * title, bool fullscreen, bool maximize,
+    bool borderless, bool resizable);
+
+  void (*shellAckConfigureIfNeeded)(void);
+
+  void (*setFullscreen)(bool fs);
+
+  bool (*getFullscreen)(void);
+
+  void (*minimize)(void);
+
+  void (*shellResize)(int w, int h);
+
+  void (*setSize)(int w, int h);
+
+  void (*getSize)(int * w, int * h);
+
+  bool (*registryGlobalHandler)(
+    void * data, struct wl_registry * registry,
+    uint32_t name, const char * interface, uint32_t version);
+
+  bool (*pollInit)(struct wl_display * display);
+
+  void (*pollWait)(struct wl_display * display, int epollFd, unsigned int time);
+}
+WL_DesktopOps;
+
+#endif