UI: Forget the views' pending requests when recreating them

WipeRequesterToken() only reset the screen's token, after CreateViews() had
already handed it to the new views, so nothing was ever forgotten - not even
in ~Screen(), which saw a token of -1. A text input dialog open across a
RecreateViews() (on the RetroAchievements screen, a slow login failing) would
then write through a deleted PopupTextInputChoice, possibly into the field
that took its place: #21708.

Requests made while creating views now get their own token, forgotten
before the views are deleted. Screen-level requests keep the screen's token.

RequestManager: ForgetRequestsWithToken() now locks, and also drops
responses that arrived but weren't processed yet. ProcessRequests() calls
the callbacks one at a time without holding the lock, so a callback that
closes a screen drops the rest of that screen's responses.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Common/UI/Screen.cpp

@@ -27,18 +27,27 @@ void Screen::focusChanged(ScreenFocusChange focusChange) {
 }
 
 int Screen::GetRequesterToken() {
-	if (token_ < 0) {
-		token_ = g_requestManager.GenerateRequesterToken();
+	int &token = creatingViews_ ? viewsToken_ : token_;
+	if (token < 0) {
+		token = g_requestManager.GenerateRequesterToken();
+	}
+	return token;
+}
+
+void Screen::ForgetViewRequests() {
+	if (viewsToken_ >= 0) {
+		g_requestManager.ForgetRequestsWithToken(viewsToken_);
+		viewsToken_ = -1;
 	}
-	return token_;
 }
 
 Screen::~Screen() {
 	screenManager_ = nullptr;
+	// To avoid expired callbacks getting called.
 	if (token_ >= 0) {
-		// To avoid expired callbacks getting called.
 		g_requestManager.ForgetRequestsWithToken(token_);
 	}
+	ForgetViewRequests();
 }
 
 const char *DialogResultToString(DialogResult result) {