Add Colorspace.BT2020 for BT.2020 YUV matrix conversion

[mark-oshea]

Add Colorspace.BT2020 for BT.2020 YUV matrix conversion

Summary

Expose libswscale's SWS_CS_BT2020 in PyAV's Colorspace enum so HDR content tagged bt2020nc can use the correct YUV→RGB matrix coefficients through VideoFrame.reformat() and VideoFrame.to_ndarray().

libswscale already supports BT.2020 matrix conversion; PyAV's Cython declarations and Python enum were missing the constant, so users could not pass BT2020 as src_colorspace / dst_colorspace.

Changes

• Declare SWS_CS_BT2020 in av/video/reformatter.pxd
• Add Colorspace.BT2020 and backward-compat alias Colorspace.bt2020 in av/video/reformatter.py
• Map SWS_CS_BT2020 → AVCOL_SPC_BT2020_NCL in _set_frame_colorspace (non-constant-luminance, the standard for consumer HDR / bt2020nc)
• Update type stubs in av/video/reformatter.pyi
• Add parametrized test test_reformat_dst_colorspace_metadata covering all distinct colorspace matrix mappings, including BT.2020
• Add changelog entry under v18.0.0 (next)

Usage

import av

# Explicit matrix selection during YUV → RGB conversion
rgb = frame.reformat(format="rgb24", src_colorspace=av.video.reformatter.Colorspace.BT2020)

# Also works via to_ndarray kwargs
arr = frame.to_ndarray(format="rgb24", src_colorspace="BT2020")

Notes

• AVCOL_SPC_BT2020_NCL was already declared in include/avutil.pxd; no FFmpeg binding changes were required.
• SWS_CS_BT2020 is the only libswscale colorspace constant not previously exposed in PyAV's Colorspace enum.

Test plan

• make — Cython build succeeds
• python -m pytest tests/test_colorspace.py -v — all tests pass, including parametrized metadata test
• python -m pytest tests/test_videoframe.py -k reformat -v — reformat regression tests pass
• make test — full suite passes locally (471 passed, 17 skipped)
• Validated against real HDR content in downstream application