Add generic GPU video post-process seam

[jeremysrunning]

Summary

• add a device-scoped generic video post-processor factory and synchronous frame context
• route FlyleafVP and D3D11VP through reusable BGRA intermediates only when configured
• share the same post-process path with snapshots and add paused-frame RequestRender invalidation
• recreate/dispose processor resources with renderer device lifetime and fail open on callback exceptions

The callback runs under renderer ownership before Direct2D/subtitle overlays. With no factory configured, the existing direct render path is unchanged and incurs no extra pass. The API contains no application- or LUT-specific concepts.

Validation

• Flyleaf master net8.0-windows Release build
• downstream invert processor exercised on FlyleafVP and hardware D3D11VP
• live presentation, paused invalidation, snapshots, stepping, source switching, exception fail-open, and disposal