Make sure the UIKit message box is being handled on the main thread

Potentially fixes https://github.com/libsdl-org/SDL/issues/4865

src/video/uikit/SDL_uikitmessagebox.m

@@ -186,8 +186,8 @@ UIKit_ShowMessageBoxAlertView(const SDL_MessageBoxData *messageboxdata, int *but
 #endif /* __IPHONE_OS_VERSION_MIN_REQUIRED < 80000 */
 }
 
-int
-UIKit_ShowMessageBox(const SDL_MessageBoxData *messageboxdata, int *buttonid)
+static void
+UIKit_ShowMessageBoxImpl(const SDL_MessageBoxData *messageboxdata, int *buttonid, int *returnValue)
 {
     BOOL success = NO;
 
@@ -199,12 +199,26 @@ UIKit_ShowMessageBox(const SDL_MessageBoxData *messageboxdata, int *buttonid)
     }
 
     if (!success) {
-        return SDL_SetError("Could not show message box.");
+        *returnValue = SDL_SetError("Could not show message box.");
+    } else {
+        *returnValue = 0;
     }
-
-    return 0;
 }
 
+int
+UIKit_ShowMessageBox(const SDL_MessageBoxData *messageboxdata, int *buttonid)
+{ @autoreleasepool
+{
+    __block int returnValue = 0;
+
+    if ([NSThread isMainThread]) {
+        UIKit_ShowMessageBoxImpl(messageboxdata, buttonid, &returnValue);
+    } else {
+        dispatch_sync(dispatch_get_main_queue(), ^{ UIKit_ShowMessageBoxImpl(messageboxdata, buttonid, &returnValue); });
+    }
+    return returnValue;
+}}
+
 #endif /* SDL_VIDEO_DRIVER_UIKIT */
 
 /* vi: set ts=4 sw=4 expandtab: */