[Feature]: Software H.264 fallback when GPU encoder MFT is unavailable

[EtienneLescot]

Summary

Follow-up to #15. When the system has no hardware H.264 Media Foundation Transform (MFT) registered (e.g. broken AMD AMF registration, missing GPU drivers, headless / no-GPU environments), OpenScreen currently fails outright. We should fall back to the Windows software H.264 encoder (mfh264enc.dll) so recording still works — slower, but functional.

Context

See #15 for the root cause analysis. Items 1 and 2 (pre-flight MFT check + better error message) are landing separately. This issue is specifically about the fallback path.

Proposed approach

1. After pre-flight enumeration (per [Bug]: Failed to initialize Media Foundation encoder - MP4 recording fails on Windows 10 with AMD GPU (hr=0x80070003) #15), if no hardware H.264 encoder MFT is found, attempt to activate the software H.264 MFT explicitly.
2. The software encoder is registered under the same category (MFT_CATEGORY_VIDEO_ENCODER) with a different CLSID and no MF_TRANSFORM_FLAG_HARDWARE attribute. MFTEnumEx returns both.
3. Add a config flag (preferSoftwareEncoder: boolean) so users can opt in permanently (e.g. on VMs or broken-driver machines) and so the smoke tests are deterministic.
4. If hardware is found but MFCreateSinkWriterFromURL still fails (the [Bug]: Failed to initialize Media Foundation encoder - MP4 recording fails on Windows 10 with AMD GPU (hr=0x80070003) #15 path), retry with the software encoder MFT explicitly selected.
5. If software also fails, surface the final clear error from [Bug]: Failed to initialize Media Foundation encoder - MP4 recording fails on Windows 10 with AMD GPU (hr=0x80070003) #15.

Acceptance criteria

• On a system with no working GPU encoder, recording to MP4 still succeeds using the software encoder.
• A new config knob lets users force software encoding.
• A wgc-helper smoke test (
  pm run test:wgc-helper:win) covers the software path.
• No regression for systems that have a working hardware encoder.

Out of scope

• Pre-flight encoder check and improved error messages — tracked in [Bug]: Failed to initialize Media Foundation encoder - MP4 recording fails on Windows 10 with AMD GPU (hr=0x80070003) #15.
• AAC audio encoder fallback (separate issue if needed).