Wayland: Add tracking of XDG fullscreen state

(cherry picked from commit afb127769a)

src/wl_platform.h

@@ -182,6 +182,7 @@ typedef struct _GLFWwindowWayland
     GLFWbool                    visible;
     GLFWbool                    maximized;
     GLFWbool                    activated;
+    GLFWbool                    fullscreen;
     GLFWbool                    hovered;
     GLFWbool                    transparent;
     struct wl_surface*          surface;

src/wl_window.c

@@ -566,6 +566,7 @@ static void xdgToplevelHandleConfigure(void* userData,
 
     window->wl.activated  = activated;
     window->wl.maximized  = maximized;
+    window->wl.fullscreen = fullscreen;
 }
 
 static void xdgToplevelHandleClose(void* userData,