assColorMatching

Match an ASS script's colours to the video the way its YCbCr Matrix header asks (#499), so a sign coloured to blend into the picture blends in here too. Typesetters pick such colours from a frame decoded with one matrix, and the header names it; a script with no header is an old VSFilter one, which counts as BT.601 at studio range. Each colour goes from RGB to YCbCr with the header's matrix and range, and back to RGB with the video's, as XySubFilter does and as libass recommends. None keeps the colours as they are, and so do RGB video and HDR video. Both the typesetting engine and the Kotlin tier follow it. False draws every colour as authored, for an application that wants exactly those.