X11: center parentless message boxes on the primary display if possible.

This relies on a successful SDL_Init(SDL_INIT_VIDEO) to work, since it's
silly to reproduce all the Xinerama/XRandR code in the message box parts. If
X11 is available but SDL hasn't been initialized, the message box will center
in the primary screen, which will be positioned weirdly on multi-head setups,
but this should fix the most significant common case.

src/video/x11/SDL_x11messagebox.c

@@ -439,8 +439,16 @@ X11_MessageBoxCreateWindow( SDL_MessageBoxDataX11 *data )
         y = attrib.y + ( attrib.height - data->dialog_height ) / 3 ;
         X11_XTranslateCoordinates(display, windowdata->xwindow, RootWindow(display, data->screen), x, y, &x, &y, &dummy);
     } else {
-        x = ( DisplayWidth( display, data->screen ) - data->dialog_width ) / 2;
-        y = ( DisplayHeight( display, data->screen ) - data->dialog_height ) / 3 ;
+        const SDL_VideoDevice *dev = SDL_GetVideoDevice();
+        if ((dev) && (dev->displays) && (dev->num_displays > 0)) {
+            const SDL_VideoDisplay *dpy = &dev->displays[0];
+            const SDL_DisplayData *dpydata = (SDL_DisplayData *) dpy->driverdata;
+            x = dpydata->x + (( dpy->current_mode.w - data->dialog_width ) / 2);
+            y = dpydata->y + (( dpy->current_mode.h - data->dialog_height ) / 3);
+        } else {   /* oh well. This will misposition on a multi-head setup. Init first next time. */
+            x = ( DisplayWidth( display, data->screen ) - data->dialog_width ) / 2;
+            y = ( DisplayHeight( display, data->screen ) - data->dialog_height ) / 3 ;
+        }
     }
     X11_XMoveWindow( display, data->window, x, y );