Wayland: Simplify test for fallback decorations

The GLFW fallback decorations only exist when the window is visible,
decorated and in windowed mode.

(cherry picked from commit 228428fa4f)

src/wl_window.c

@@ -2060,7 +2060,7 @@ void _glfwPlatformGetWindowFrameSize(_GLFWwindow* window,
                                      int* left, int* top,
                                      int* right, int* bottom)
 {
-    if (window->decorated && !window->monitor && window->wl.decorations.top.surface)
+    if (window->wl.decorations.top.surface)
     {
         if (top)
             *top = GLFW_CAPTION_HEIGHT;