x11: Don't let XRRSetScreenSize fire a BadMatch error.

This is a workaround and not a proper fix, but this is possibly complicated,
and possibly a corner case, so this will do for 2.0.16, if not the
foreseeable future.

Reference issue #4561

src/video/x11/SDL_x11modes.c

@@ -995,6 +995,15 @@ X11_GetDisplayModes(_THIS, SDL_VideoDisplay * sdl_display)
     }
 }
 
+/* This catches an error from XRRSetScreenSize, as a workaround for now. */
+/* !!! FIXME: remove this later when we have a better solution. */
+static int (*PreXRRSetScreenSizeErrorHandler)(Display *, XErrorEvent *) = NULL;
+static int
+SDL_XRRSetScreenSizeErrHandler(Display *d, XErrorEvent *e)
+{
+    return (e->error_code == BadMatch) ? 0 : PreXRRSetScreenSizeErrorHandler(d, e);
+}
+
 int
 X11_SetDisplayMode(_THIS, SDL_VideoDisplay * sdl_display, SDL_DisplayMode * mode)
 {
@@ -1040,7 +1049,17 @@ X11_SetDisplayMode(_THIS, SDL_VideoDisplay * sdl_display, SDL_DisplayMode * mode
 
         mm_width = mode->w * DisplayWidthMM(display, data->screen) / DisplayWidth(display, data->screen);
         mm_height = mode->h * DisplayHeightMM(display, data->screen) / DisplayHeight(display, data->screen);
+
+        /* !!! FIXME: this can get into a problem scenario when a window is
+           bigger than a physical monitor in a configuration where one screen
+           spans multiple physical monitors. A detailed reproduction case is
+           discussed at https://github.com/libsdl-org/SDL/issues/4561 ...
+           for now we cheat and just catch the X11 error and carry on, which
+           is likely to cause subtle issues but is better than outright
+           crashing */
+        PreXRRSetScreenSizeErrorHandler = X11_XSetErrorHandler(SDL_XRRSetScreenSizeErrHandler);
         X11_XRRSetScreenSize(display, RootWindow(display, data->screen), mode->w, mode->h, mm_width, mm_height);
+        X11_XSetErrorHandler(PreXRRSetScreenSizeErrorHandler);
 
         status = X11_XRRSetCrtcConfig (display, res, output_info->crtc, CurrentTime,
           crtc->x, crtc->y, modedata->xrandr_mode, crtc->rotation,