When support for this was added the details weren't yet finalized.
This is no longer the case.
Fixes writing of mkv/webm files with HDR.
Reported-by: Kagami Hiiragi <kagami@genshiken.org>
Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>
Reviewed-by: James Almer <jamrial@gmail.com>