feat: IOSurface keepalive for static screens

[lukemarsden]

Summary

• Adds a keepalive thread that re-encodes the last blitted IOSurface to VideoToolbox when no page flips arrive for 500ms
• Produces valid H.264 frames with proper frame_num sequencing and correct reference lists
• Replaces the previous approach in desktop-bridge which re-sent already-encoded P-frames, corrupting the decoder's DPB (Decoded Picture Buffer)

How it works

When the guest screen is static, virtio_gpu_cmd_resource_flush stops firing and the encoder goes idle. The keepalive thread:

1. Checks every 250ms if any scanout has been idle for >500ms
2. If idle and clients are subscribed, wraps the last IOSurface[last_blit_slot] as a new CVPixelBuffer (zero-copy)
3. Submits to VTCompressionSessionEncodeFrame with a new PTS (continuing the monotonic sequence)
4. VideoToolbox produces a proper P-frame that references the previous frame correctly

No GL blit is needed — the IOSurface still holds the last raw pixel data. The keepalive respects all existing safety mechanisms (vt_busy, blit_slot_busy, fe->mutex).

Files changed

• hw/display/helix/helix-frame-export.h — Added keepalive fields to HelixScanoutEncoder, HELIX_KEEPALIVE_INTERVAL_MS constant
• hw/display/helix/helix-frame-export.m — Keepalive thread, tracking in helix_scanout_frame_ready(), init/reset

Test plan

• Build QEMU with changes
• Start a VM and connect a streaming client
• Verify frames continue at ~2 FPS when the screen is static
• Verify no H.264 decoder corruption (clean P-frames, no visual artifacts)
• Verify keepalive stops when no clients are connected
• Verify normal 60 FPS encoding still works during active rendering

🤖 Generated with Claude Code

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