sd_ass: add `sub-vsfilter-bidi-compat` to enable vsfilter bidi compatHEADmaster

Enable ASS_FEATURE_{WHOLE_TEXT_LAYOUT, BIDI_BRACKETS} and auto base
detection by default, and add an option to disable this if needed.

This is strictly an improvement for webvtt files as they always use
auto base detection. This _fixes_ right-to-left text rendering for
webvtt files which correctly mark rtl/ltr. Webvtt files obtained from
sources which sideload the RTL information through css also see an
improvement due to the auto detection.

Generally SRT files also want this, but some are also written to
workaround VSFilter quirks.

See also: https://github.com/mpv-player/mpv/pull/12985#issuecomment-1839565138

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