Implemented workaround for VapourSynth plug-in loading: Testing shows that the enviro...

Implemented workaround for VapourSynth plug-in loading: Testing shows that the environment variable %USERPROFILE% must be set correctly; otherwise VapourSynth fails to load plugins from the "<AppData>\VapourSynth\plugins{32|64}" directory.
Even though it would make more sense to evaluate %APPDATA%, VapourSynth apparently ignores that one. We set up %APPDATA% anyway, just to be sure.