Attempt to reduce leak of ffmpeg previews

cista/preview.py

@@ -1,4 +1,5 @@
 import asyncio
+import gc
 import io
 import mimetypes
 import urllib.parse
@@ -105,10 +106,13 @@ def process_video(path, *, maxsize, quality):
         container.seek(container.duration // 8)
         frame = next(container.decode(stream))
         img = frame.to_image()
+        del frame, stream
 
     img.thumbnail((maxsize, maxsize))
     imgdata = io.BytesIO()
     if rotation:
         img = img.rotate(rotation, expand=True)
     img.save(imgdata, format="webp", quality=quality, method=4)
-    return imgdata.getvalue()
+    ret = imgdata.getvalue()
+    gc.collect()
+    return ret