Recording (macOS): capture and tag BT.709 colour

[EtienneLescot]

Part of #920 (rank 16). Effort S, risk low.

Problem

• The compositor decodes every recording as BT.709 studio range. Windows (fix(windows): encode H.264 High with the BT.709 colour the compositor expects #929) and Linux (fix(linux): convert and tag the PipeWire helper's video as BT.709 #932) were found writing BT.601; macOS has never been measured.
• ScreenCaptureKit captures kCVPixelFormatType_32BGRA with no colorSpaceName, and the writer sets no AVVideoColorPropertiesKey, so VideoToolbox picks the matrix and the tags (ScreenCaptureRecorder.swift). A P3 display adds a gamut question on top.
• BGRA also costs 4 bytes a pixel with queueDepth = 6: about 200 MB of buffers at 4K.

Proposal

• Ask ScreenCaptureKit for sRGB (colorSpaceName) and set AVVideoColorPropertiesKey to BT.709 primaries, transfer and matrix on the writer.
• If it holds up, capture kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange with a BT.709 colorMatrix, which also divides buffer memory by about 2.7.

Verify

• On a Mac: ffprobe -show_streams | grep color_ reads tv / bt709 / bt709 / bt709; solid red read back as raw YUV gives Y≈63, Cb≈102, Cr≈240.

[github-actions]

Fixed by #947 and merged into main.

This change is assigned to the Next Release release milestone and is not necessarily available in the latest downloadable desktop release yet. It is currently marked as status: pending release until a packaged release containing it is published.

[EtienneLescot]

Moved into v2.0.0 as part of the pre-release cut.