feat(encoder): Add VideoToolbox hardware H.264/H.265 encoder for Apple platforms

[jspanchu]

On Apple Silicon the only working encoder was software VP9; NVENC is NVIDIA-only. Add a VTKStreaming::VTEncode module whose vtkVideoToolboxEncoder subclasses vtkVideoEncoder and drives a VTCompressionSession on the hardware media engine with https://developer.apple.com/documentation/videotoolbox?language=objc, encoding H.264 and HEVC.

Details:

• CPU-uploads the captured NV12/IYUV/RGBA32 frame into an IOSurface-backed CVPixelBuffer from the session pool, then hardware-encodes it.
• Emits an Annex B elementary stream (start codes, parameter sets inlined on key frames), byte-compatible with the NVENC backend and directly playable by ffplay.
• Reports a full RFC 6381 / WebCodecs codec string (avc1./hvc1.) parsed from the SPS, mirroring the NVENC backend's BuildCodecString.
• Honors BitRateControlMode: CBR (ConstantBitRate on macOS 13+, else average bitrate + data-rate cap), VBR (average + ceiling). VideoToolbox has no constant-QP mode, so CQP/QP map QuantizationParameter to a quality hint plus allowed-QP bounds.
• Module is only added to the build under if(APPLE); non-Apple builds are unaffected.

Adds a Python test (requires_videotoolbox helper), an example, and updates the README.

[jspanchu]

newly added videotoolbox tests pass https://github.com/Kitware/VTKStreaming/actions/runs/29048565697/job/86223212457?pr=5#step:5:1105

../../../../../../../../../Users/runner/work/VTKStreaming/VTKStreaming/tests/test_videotoolbox_encode_simple.py::test_videotoolbox_encode_simple[iyuv] PASSED [ 60%]
../../../../../../../../../Users/runner/work/VTKStreaming/VTKStreaming/tests/test_videotoolbox_encode_simple.py::test_videotoolbox_encode_simple[nv12] PASSED [ 66%]
../../../../../../../../../Users/runner/work/VTKStreaming/VTKStreaming/tests/test_videotoolbox_encode_simple.py::test_videotoolbox_encode_simple[rgba32] PASSED [ 73%]

[jourdain]

Next steps:

• Make sure all classes exist regardless of platform. The CheckAvailability() can always return false when not supported on that platform.
• Create factory implementation relying on vtk 9.7 factory preference mechanism with a VTK 9.6 path that just ignore the preference request.

[jspanchu]

Next steps:

• Make sure all classes exist regardless of platform. The CheckAvailability() can always return false when not supported on that platform.
• Create factory implementation relying on vtk 9.7 factory preference mechanism with a VTK 9.6 path that just ignore the preference request.

On item 1: I ended up not needing this. The vtkEncoderFactory makes it moot. Consumers never import a concrete encoder class, they only go through the factory. Each backend self-registers into the factory's registry at load time, so on a given platform the registry simply contains whatever was built (VTEncode stays Apple-only, NvEncode/Vpx build everywhere). CheckAvailability(codec) and CreateEncoder() then degrade gracefully in two ways: a backend that isn't built is absent from the registry, and one that's built-but-unusable returns false from its own CheckAvailability() probe. So there's no missing-symbol risk and no need to force every class onto every platform.

On step 2 (9.6 path): this lives in vtkEncoderFactory. One tweak from what we sketched, instead of ignoring the preference request on 9.6, the interim factory actually honors it: it takes the same VTK 9.7 preference string ("Codec=H265,H264,VP9;Hardware=true", ; between keys and , between ranked values) and computes a lexicographic rank over the registered backends. It reuses the 9.7 attribute vocabulary I added throughout, so the migration will be simple: swap vtkEncoderFactory::SetPreferences/CreateEncoder for vtkObjectFactory::SetPreferences/vtkVideoEncoder::New() and promote each backend's attribute map into CreateOverrideAttributes(), then delete the helper.

[jourdain]

LGTM