Secondary outputs, and --game-window on Mac to show the game in a second window

The ScreenManager keeps a list of secondary outputs, each with its own
swapchain, recreated after a lost device. Platform code adds, resizes and
removes them from any thread through Native_AddOutput() and friends, which
take effect at the start of the next frame.

Each frame, every output is drawn by the topmost screen that claims it
(Screen::renderOutput). EmuScreen claims Game outputs: it presents the
game there, laid out for that output, and leaves it out of the backbuffer,
where the touch controls and the menus stay. Not in non-buffered mode,
which draws straight into the backbuffer.

--game-window opens such an output on Mac, with Metal. It's a smoke test
for multiple swapchains, and a stand-in for an AirPlay display.

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

Core/CmdLine.cpp

@@ -189,6 +189,7 @@ static const CommandLineParam g_autoParams[] = {
 	{POFF(fullscreen), CmdParamType::BoolInverse, "windowed", '\0', "Force windowed mode", CmdLineMode::Application},
 	{POFF(startScreen), CmdParamType::String, "start-screen", '\0', "Start on a specific screen (e.g. 'gamesettings', 'touchscreentest')", CmdLineMode::Application},
 	{POFF(escapeExit), CmdParamType::Bool, "escape-exit", '\0', "Escape key exits the application", CmdLineMode::Application},
+	{POFF(gameWindow), CmdParamType::Bool, "game-window", '\0', "Show the game in a second window, for testing multiple outputs (SDL, Metal only)", CmdLineMode::Application},
 	{POFF(pauseMenuExit), CmdParamType::Bool, "pause-menu-exit", '\0', "Exit the application when the game ends, instead of returning to the menu", CmdLineMode::Application},
 	{POFF(appendConfig), CmdParamType::String, "appendconfig", '\0', "Merge config FILE into the current configuration"},
 	{POFF(root), CmdParamType::String, "root", 'r', "Mount directory as the root of host0:/."},