X11: Filter out Xlib errors from other connections

(cherry picked from commit 2c8e0512dd)

README.md

@@ -119,6 +119,8 @@ information on what to include when reporting a bug.
 ## Changelog
 
  - [X11] Bugfix: IME input of CJK was broken for "C" locale (#1587,#1636)
+ - [X11] Bugfix: Xlib errors caused by other parts of the application could be
+   reported as GLFW errors
 
 
 ## Contact

src/x11_init.c

@@ -851,6 +851,9 @@ static Window createHelperWindow(void)
 //
 static int errorHandler(Display *display, XErrorEvent* event)
 {
+    if (_glfw.x11.display != display)
+        return 0;
+
     _glfw.x11.errorCode = event->error_code;
     return 0;
 }