Implementation of Remaining Metal Backend

[rg2]

Summary

Adds Metal implementations of the 2D similarity metrics and of the surface rendering and occluding contour ray casters. With these, a registration can run on a Mac GPU with Metal for both ray casting and similarity scoring. Metal becomes the default backend when a Metal device is available.

New Metal implementations

2D similarity metrics (lib/regi/sim_metrics_2d/)

• SSD, NCC, Grad-NCC, Patch NCC and Patch Grad-NCC (ImgSimMetric2D*Metal), with a shared base class ImgSimMetric2DMetal.
• They follow the OpenCL implementations' semantics: mask handling, Gaussian smoothing, edge handling and patch weighting. They don't support the CPU-only Patch NCC options.
• --backend metal works for all five *SimMetricFromProgOpts() factories.

Ray casters (lib/ray_cast/)

• RayCasterSurRenderMetal: Phong-shaded surface rendering.
• RayCasterOccludingContoursMetal: occluding contours.

Shared Metal infrastructure

• MetalComputeEncoder (xregMetalCompute.h): a plain C++ wrapper for encoding several dependent kernel dispatches and waiting once, so metric and caster classes don't need Objective-C++.
• ThreadgroupReducer (xregMetalReduce.metal): sums across a threadgroup using SIMD-group reductions.
• BacktrackToSurface and CentralDiffGradient in xregRayCastBaseMetal.metal. The existing depth kernel now uses BacktrackToSurface; its results are unchanged.

Bug fixes in existing code

• ImgSimMetric2DSSDOCL: kernel arguments were set before the fixed-image and mask buffers existed. With a host fixed image this crashed; otherwise the image length argument was 0, so the result was each moving image's mean intensity instead of the SSD.
• RayCasterSurRenderOCL:
  • one extra backtracking step was taken, even with 0 steps requested;
  • backtracking moved in the wrong direction;
  • the surface normal had length 2, which doubled the diffuse term and distorted the specular term.
• RayCasterSurRenderCPU:
  • the volume's upper index bound was never initialized (std::tie(min, min));
  • an out-of-bounds write to a Pt2 divided every shaded value by the camera focal length.
• RayCasterOccludingContoursCPU: when continuing past a collision with backtracking, the ray resumed from the backtracked position and stopped advancing inside objects. It now resumes from the sample.

Behaviour changes

• Default backend: Metal is now the default when a Metal device is available. The backend order is Metal, then OpenCL, then CPU, so systems without Metal are unaffected. --backend ocl and --backend cpu still work.
• --ocl-id: now accepts any case-insensitive substring that matches exactly one OpenCL device ID, the same as --metal-id. No match, or more than one, is an error that lists the IDs. An exact full ID still takes precedence. The matching logic is shared via FindMapEntriesByKeySubstrIgnoreCase (xregStringUtils.h).
• tests/wiki_cmds.py: new --backend and --device options.