Attempt to reduce leak of ffmpeg previews

cista/preview.py

@@ -98,8 +98,9 @@ def process_pdf(path, *, maxsize, maxzoom, quality, page_number=0):
 def process_video(path, *, maxsize, quality):
     with av.open(str(path)) as container:
         stream = container.streams.video[0]
-        rot = stream.side_data and stream.side_data.get(SideData.DISPLAYMATRIX) or 0
         stream.codec_context.skip_frame = "NONKEY"
+        stream.codec_context.threads = 1
+        rot = stream.side_data and stream.side_data.get(SideData.DISPLAYMATRIX) or 0
         container.seek(container.duration // 8)
         img = next(container.decode(stream)).to_image()
         del stream