PGS subtitles should use same colorspace as video
If PGS subtitles are embedded in a video using the BT.709 colorspace, it should be assumed that those subtitles also use that colorspace (rather than RGB).
Under normal circumstances, this is not noticeable, but some SDH-style subtitles use a solid black background. When encoded as BT.709 black (16), VLC currently displays the background as dark gray, as if it were RGB. The dark gray background is actually brighter than the background under some circumstances and can be distracting.