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>

GPU/Common/FramebufferManagerCommon.cpp

@@ -128,6 +128,10 @@ bool FramebufferManagerCommon::PresentedThisFrame() const {
 	return presentation_->PresentedThisFrame();
 }
 
+void FramebufferManagerCommon::SetPresentationOutput(const OutputParams *output) {
+	presentation_->SetOutputOverride(output);
+}
+
 void FramebufferManagerCommon::SetDisplayFramebuffer(u32 framebuf, u32 stride, GEBufferFormat format) {
 	displayFramebufPtr_ = framebuf & 0x3FFFFFFF;
 	if (Memory::IsVRAMAddress(displayFramebufPtr_))