X11: Fix BadMatch focusing a window on non-EWMH WM

When the WM does not support EWMH or there is no WM running, GLFW falls
back to XSetInputFocus, which will emit BadMatch if the window is not
viewable, which will terminate the program.

Bug spotted on IRC.

README.md

@@ -158,6 +158,7 @@ information on what to include when reporting a bug.
  - [X11] Bugfix: Disabled cursor mode was interrupted by indicator windows
  - [X11] Bugfix: Monitor physical dimensions could be reported as zero mm
  - [X11] Bugfix: Window position events were not emitted during resizing (#1613)
+ - [X11] Bugfix: `glfwFocusWindow` could terminate on older WMs or without a WM
  - [Wayland] Removed support for `wl_shell` (#1443)
  - [Wayland] Bugfix: The `GLFW_HAND_CURSOR` shape used the wrong image (#1432)
  - [Wayland] Bugfix: `CLOCK_MONOTONIC` was not correctly enabled

src/x11_window.c

@@ -2457,7 +2457,7 @@ void _glfwPlatformFocusWindow(_GLFWwindow* window)
 {
     if (_glfw.x11.NET_ACTIVE_WINDOW)
         sendEventToWM(window, _glfw.x11.NET_ACTIVE_WINDOW, 1, 0, 0, 0, 0);
-    else
+    else if (_glfwPlatformWindowVisible(window))
     {
         XRaiseWindow(_glfw.x11.display, window->x11.handle);
         XSetInputFocus(_glfw.x11.display, window->x11.handle,