vf_vapoursynth: improve backward compatibility

The previous commit to dynamically load VapourSynth R74 would fail to work
for VapourSynth R73 and before on Linux due to load mode RTLD_LOCAL. To
make it work on all VapourSynth version, switch to use RTLD_GLOBAL mode.

Add library handle closing in drv_vss_uninit for completeness.