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
@@ -109,5 +110,7 @@ def process_video(path, *, maxsize, quality):
         img = img.rotate(rot, expand=True)
     img.save(imgdata, format="webp", quality=quality, method=4)
     del img
-
-    return imgdata.getvalue()
+    ret = imgdata.getvalue()
+    del imgdata
+    gc.collect()
+    return ret