On Windows, plugins are shipped with Aegisub now and loaded from
aegisub_vs.py, so user plugins installed out of Aegisub are more
likely to just interfere with that (like when the user has an
old version of libvslsmashsource installed).
So this option defaults to off on Windows, but to on everywhere else
since VS plugins aren't shipped with Aegisub there.
Note that this option only disables autoloading of user plugins.
VapourSynth loads system plugins no matter what.
arch1t3cht