External display setting: Show the game, or mirror the screen

Graphics > "External display" on iOS and Android. "Show the game" (the
default) is the TV mode, "Mirror the screen" leaves the TV to the system.
A change applies right away. It's disabled on backends that can't draw to
a second swapchain.

On Android 12 and up, a second screen that's part of the device (like a
dual-screen handheld's) isn't used, so the game doesn't move there by
itself.

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

Core/Config.cpp

@@ -749,6 +749,7 @@ static int DefaultAnalogAltSpeedPercent() {
 static constexpr ConfigSetting graphicsSettings[] = {
 	ConfigSetting("iShowStatusFlags", SETTING(g_Config, iShowStatusFlags), 0, CfgFlag::PER_GAME),
 	ConfigSetting("GraphicsBackend", SETTING(g_Config, iGPUBackend), &DefaultGPUBackend, &GPUBackendTranslator::To, &GPUBackendTranslator::From, CfgFlag::DEFAULT | CfgFlag::REPORT),
+	ConfigSetting("ExternalDisplay", SETTING(g_Config, iExternalDisplay), (int)ExternalDisplayMode::SHOW_GAME, CfgFlag::DEFAULT),
 #if PPSSPP_PLATFORM(ANDROID) && PPSSPP_ARCH(ARM64)
 	ConfigSetting("CustomDriver", SETTING(g_Config, sCustomDriver), "", CfgFlag::DEFAULT),
 #endif