X11: Fix previous error handler not being restored

GLFW did not restore the previous Xlib error handler when removing its
own, instead resetting to the default handler.

This commit saves and restores the previous error handler.

None of this is thread-safe or could ever be.

Fixes #2108

src/x11_platform.h

@@ -567,6 +567,8 @@ typedef struct _GLFWlibraryX11
     XContext        context;
     // XIM input method
     XIM             im;
+    // The previous X error handler, to be restored later
+    XErrorHandler   errorHandler;
     // Most recent error code received by X error handler
     int             errorCode;
     // Primary selection string (while the primary selection is owned)