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>

Core/CmdLine.cpp

@@ -468,6 +468,9 @@ CommandLineParseResult CommandLineOptions::Parse(int argc, const char *argv[], C
 			} else if (restOfOption == "vulkan") {
 				gpuBackend = GPUBackend::VULKAN;
 				softwareRendering = false;
+			} else if (restOfOption == "metal") {
+				gpuBackend = GPUBackend::METAL;
+				softwareRendering = false;
 			} else if (restOfOption == "software") {
 				gpuBackend = GPUBackend::OPENGL;
 				softwareRendering = true;