vf_vapoursynth: reset error state on seeking

When seeking, we violently destroy the filter, because vapoursynth has
no proper API for terminating a video with unknown frame count. This
looks like an error to vapoursynth, and the error is returned via the
frame callbacks. The bug is that we remember this error state across
reinitialization, so on the first filter call after reinitialization, we
thought filtering the current frame failed. This caused a shift by 1
frame on each seek.

CC: @mpv-player/stable

video/filter/vf_vapoursynth.c

@@ -434,6 +434,7 @@ static void destroy_vs(struct vf_instance *vf)
     p->next_image = NULL;
     p->out_pts = MP_NOPTS_VALUE;
     p->out_frameno = p->in_frameno = 0;
+    p->failed = false;
 
     MP_DBG(vf, "uninitialized.\n");
 }