Wayland: Do not decorate fullscreen windows

If a fullscreen window with GLFW_DECORATED set had its XDG decorations
changed to client mode by the compositor, it would seemingly receive
GLFW fallback decorations as if it was windowed mode.

This is possibly related to #2001.

(cherry picked from commit da6713cd09)

src/wl_window.c

@@ -319,7 +319,7 @@ static void xdgDecorationHandleConfigure(void* userData,
 
     if (mode == ZXDG_TOPLEVEL_DECORATION_V1_MODE_CLIENT_SIDE)
     {
-        if (window->decorated)
+        if (window->decorated && !window->monitor)
             createFallbackDecorations(window);
     }
     else