Wayland: Fix missing damage event on window show

By definition a hidden window on Wayland does not have valid framebuffer
contents.

This adds a window damage (refresh) event when a window is shown, to
request an initial frame for the now visible window.

(cherry picked from commit 25c521cbe5)

src/wl_window.c

@@ -1180,6 +1180,7 @@ void _glfwPlatformShowWindow(_GLFWwindow* window)
             createShellSurface(window);
 
         window->wl.visible = GLFW_TRUE;
+        _glfwInputWindowDamage(window);
     }
 }