Metal: Hook up the backend on Mac, with the null GPU for now

Selectable with --graphics=metal, the backend choice in settings and the
Mac menu bar. SDL creates a Metal view for it, and headless renders
offscreen. Games run on the null GPU until there's a GPU/Metal.

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

UI/EmuScreen.cpp

@@ -312,6 +312,9 @@ void EmuScreen::ProcessGameBoot(const Path &filename) {
 		coreParam.gpuCore = GPUCORE_VULKAN;
 		break;
 #endif
+	case GPUBackend::METAL:
+		coreParam.gpuCore = GPUCORE_METAL;
+		break;
 	}
 
 	// Preserve the existing graphics context.