R54.A0 First R54 Test Release

For internal testing purposes only. Internal testing completed, ready for public consumption.

Changes compared to upstream api3 branch:

• Reverted version number back to R54 to avoid any confusion
• backported std.PreMultiply and std.MaskedMerge(premultiplied=1) fixes from R55
• Tweaked cache update frequency
• Reduced initial cache size for regular filters

The result is a R54 mod that is both faster than stock R55 and use less memory on real scripts (but note that the default max cache size is 4GB, so without reducing max cache size, the memory consumption won't go lower than 4GB.)

For imwri plugin, please use https://github.com/AmusementClub/vapoursynth-classic/releases/tag/R54.A.
Thanks @Masaiki for the build of subtext with libass 0.15.2 + freetype 2.10.4 (link).

Requires Python 3.9.