graphics/libplacebo: update to 4.208.0

Changes: https://code.videolan.org/videolan/libplacebo/-/tags/v4.208.0
Changes: https://code.videolan.org/videolan/libplacebo/-/compare/v4.208.0-rc1...v4.208.0
Reported by: Repology

(cherry picked from commit a7c3c1935cb16945ceb229c278ee5cad1ff28a2a)