Overlay: follow display offset/scale, avoid needless reloads

- Apply the display layout's scale and offset to the overlay itself instead
  of always centering it, so moving/resizing the display in Display Layout
  moves the overlay with the game inside it, and the portrait default
  (screen high up) is respected.
- Rescanning overlays (done whenever the display layout screen recreates its
  views, e.g. after closing the picker popup) no longer unloads the active
  overlay unless its definition changed.
- Re-picking the current overlay no longer posts GPU config change
  notifications, which caused an audio glitch with a game running behind.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QGBg32EFPsTXjAFu7dnTcz

UI/DisplayLayoutScreen.cpp

@@ -370,7 +370,11 @@ void DisplayLayoutScreen::CreateViews() {
 		}
 		if (overlayIds.size() > 1) {
 			auto overlayChoice = leftColumn->Add(new PopupMultiChoiceDynamic(&config.sOverlay, gr->T("Overlay"), overlayNames, I18NCat::NONE, screenManager(), &overlayIds));
-			overlayChoice->OnChoice.Add([](UI::EventParams &) {
+			overlayChoice->OnChoice.Add([](UI::EventParams &e) {
+				// e.b is set if the value actually changed. Re-picking the current one shouldn't
+				// cause a reload, since that glitches the audio of a game running in the background.
+				if (!e.b)
+					return;
 				// The game rect may have moved, which matters for post shaders that run at output resolution.
 				NotifyPostChanges();
 			});