bestsource: Bump to R4

[0tkl]

Since 900876c there’s quite a lot progress in the upstream, especially the meson.build got overhauled. So removing avs/vs plugin by patch are no longer needed since vapoursynth/bestsource@477a7f5; and the link_static option become prefer_static since vapoursynth/bestsource@d59b689.

I'm a total amateur to all the decoding internals so the only thing I check is API changes. AFAIK the only difference after 900876c is a new cachemode parameter introduced in vapoursynth/bestsource@ae44622.

[arch1t3cht]

Thanks for the PR, but soon after the commit bestsource is currently pinned to, the headers started to use std::filesystem::path, which needs C++17 (while arch1t3cht/Aegisub is still C++11). So this can't be merged right now (and IMO the differences from the current commit to R2 aren't big enough to warrant hacks like making a small C++17 wrapper library to work around this).

TypesettingTools/Aegisub has already switched to C++20, and I plan to eventually figure out how to rebase my changes on top of that - when that's done this PR can be merged.

[0tkl]

Merged in 0a01ecf