iOS: Show the game on an AirPlay TV, with the controls on the phone

An external display (like an Apple TV over AirPlay) becomes a Game
output, when the backend is Metal. The game is shown there full screen,
and the phone shows the touch controls with "Playing on TV" in place of
the game. The menus stay on the phone. With other backends, iOS keeps
mirroring.

Up to iOS 26 the external display scene comes from the Info.plist role.
iOS 27 ignores that, and only offers the scene to apps that register a
scene accessory for it, so we do that too. The registration is withdrawn
when the view controller shuts down, so it follows a backend change, and
a restart now goes to the app's scene rather than the first one.

An Apple TV 4K only offers 3840x2160 over AirPlay, so the stream stays
4K, but we render at 1920x1080 and let the layer scale it up.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

UI/EmuScreen.cpp

@@ -1577,6 +1577,16 @@ ScreenRenderRole EmuScreen::renderRole(bool isTop) const {
 	return role;
 }
 
+// In place of the game, while it's on another output.
+void EmuScreen::drawGameOnOutputMessage() {
+	UIContext &dc = *screenManager()->getUIContext();
+	auto di = GetI18NCategory(I18NCat::DIALOG);
+	const Bounds &bounds = dc.GetBounds();
+	dc.Begin();
+	dc.DrawText(di->T("Playing on TV"), bounds.centerX(), bounds.centerY(), 0xFF505050, ALIGN_CENTER);
+	dc.Flush();
+}
+
 void EmuScreen::darken() {
 	if (screenManager()->topScreen()->wantBrightBackground()) {
 		return;
@@ -1714,7 +1724,9 @@ ScreenRenderFlags EmuScreen::render(ScreenRenderMode mode) {
 	Viewport viewport{0.0f, 0.0f, (float)g_display.pixel_xres, (float)g_display.pixel_yres, 0.0f, 1.0f};
 
 	if (!skipBufferEffects_ && !ShouldRunEmulation(mode)) {
-		if (gpu && !gameOnOutput_) {
+		if (gameOnOutput_) {
+			drawGameOnOutputMessage();
+		} else if (gpu) {
 			gpu->CopyDisplayToOutput(displayLayoutConfig);
 		}
 		draw->SetViewport(viewport);
@@ -1747,7 +1759,9 @@ ScreenRenderFlags EmuScreen::render(ScreenRenderMode mode) {
 	const bool skipBufferEffects = skipBufferEffects_;
 
 	// Gotta copy the output at some point. Also this is where we take the screenshot if needed.
-	if (gpu && !gameOnOutput_) {
+	if (gameOnOutput_) {
+		drawGameOnOutputMessage();
+	} else if (gpu) {
 		gpu->CopyDisplayToOutput(displayLayoutConfig);
 	}