Add Metal GPU depth packet processor backend

[mcakyurek]

Adds a new packet pipeline using Apple's Metal GPU compute framework for depth decoding on macOS, following the same structure as the existing OpenCL and CUDA backends.

New files:

• src/metal_depth_packet_processor.mm -- Host-side Objective-C++ implementation: device init, pipeline compilation, buffer management, and the four-stage dispatch loop (processPixelStage1, filterPixelStage1, processPixelStage2, filterPixelStage2). Loads default.metallib from alongside the dylib, falling back to the main bundle and device default library.
• src/metal_depth_packet_processor.metal -- Metal Shading Language compute kernels ported from the OpenCL implementation. Uses non-uniform dispatchThreads for clean IMAGE_SIZE dispatch.

Modified files:

• CMakeLists.txt: ENABLE_METAL option (ON by default on Apple), xcrun metal/metallib custom commands to compile the Metal library, MetalShaders target, framework link flags, and install rule for default.metallib.
• include/libfreenect2/config.h.in: LIBFREENECT2_WITH_METAL_SUPPORT cmake define.
• include/libfreenect2/packet_pipeline.h: MetalPacketPipeline class declaration under LIBFREENECT2_WITH_METAL_SUPPORT guard.
• include/internal/libfreenect2/depth_packet_processor.h: MetalDepthPacketProcessor class declaration.
• src/packet_pipeline.cpp: MetalPacketPipeline constructor/destructor.
• src/libfreenect2.cpp: "metal" pipeline name in createPacketPipelineByName(); Metal chosen as default on Apple when available.