Hope to change subtitle peak luminosity throught `--hdr-compute-peak` when playing pq transfer video

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Expected behavior of the wanted feature

When I played HDR videos with pq transfer function(and the windows system entered HDR mode), I found that the subtitles would be first converted to linear light space and then mapped to pq using min[203, target-trc] as the reference peak luminosity. I know that this behaviour reference the standard bt.2390. But the constant peak may cause a few problems. Because the ass file was not designed for HDR playback at first, so the style brightness value would be constant from beginning to end. This may cause the subtitle looks too bright in dark scene or too sombre in shiny scene. And in Aegisub, it is too troublesome to specify a style for each scene. Moreover, we can't make subtitle brighter than 203nit even though it looks sombre.
So I hope to add an option that the reference peak luminosity of subtitle rendering can be obtain from --hdr-compute-peak, so it brightness will vary automaticlly in the different scene.

Alternative behavior of the wanted feature

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Start by tracing the existing --hdr-compute-peak option and the subtitle rendering path for PQ-transfer video. Compare the current fixed reference peak behavior with the requested scene-dependent behavior, including ASS subtitle styles and BT.2390 mapping. Done means the reference peak can follow the computed HDR peak without breaking subtitle rendering.