QuadraH264.roiEnable property

Enable per-frame region-of-interest (ROI) encoding, for regions supplied via streamMetadataOverride’s `roi. Off by default: an ROI session reserves and transfers a per-macroblock QP map with every frame (32KB at 1080p) whether or not any regions arrive, and it cannot be switched on after the encoder has started. Supplying regions to an encoder without it is an error.

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