Add AMD GPU support for AI Green Screen via DirectML

[RahnRazamai]

Title

Add AMD GPU support for AI Green Screen via DirectML

Summary

Add a non-NVIDIA backend for AI Green Screen so AMD GPU users can use background removal without NVIDIA Maxine.

Motivation

Camera-on-Screen currently gates AI Green Screen through the NVIDIA Maxine path. The plain overlay works on non-RTX machines, but AMD users cannot use AI effects. DirectML + ONNX Runtime should let us run a segmentation model on DirectX 12-capable AMD GPUs.

Proposed Scope

• Add an optional DirectML/ONNX Runtime green-screen backend.
• Keep the existing NVIDIA Maxine green-screen path unchanged.
• Capability probing should enable Green Screen if either Maxine or DirectML backend is available.
• Add provider-specific detail text, e.g. NVIDIA Maxine or DirectML.
• Update docs to say Green Screen can run on NVIDIA Maxine or DirectML-supported GPUs.

Out of Scope

• AMD Eye Contact.
• AMD Super Resolution.
• Frame interpolation / Smooth 60 fps.
• Replacing the existing NVIDIA Maxine implementation.

Implementation Notes

Likely files:

• native/shim/aigs.*
• native/shim/capture.*
• native/shim/shim.*
• native/shim/shim.vcxproj
• src/CameraOnScreen.Core/Native/Contracts.cs
• src/CameraOnScreen.Core/Orchestration/Orchestrator.cs
• README / CONTRIBUTING / THIRD-PARTY-NOTICES

Potential backend:

• ONNX Runtime + DirectML
• Segmentation model producing an alpha matte
• Existing BGRA in/out frame contract should remain unchanged

Acceptance Criteria

• On NVIDIA RTX, existing Maxine green screen still works.
• On AMD GPU with DirectML backend available, Green Screen toggle enables.
• On unsupported machines, Green Screen remains disabled with a clear reason.
• Stub/no-SDK builds still compile.
• Core unit tests cover capability probing and effect gating.
• Docs and third-party notices are updated.