avcodec/screenpresso: return the packet size instead of 0

Most decoders return the amount of data used.
This is more consistent

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/screenpresso.c

@@ -179,7 +179,7 @@ static int screenpresso_decode_frame(AVCodecContext *avctx, void *data,
     }
     *got_frame = 1;
 
-    return 0;
+    return avpkt->size;
 }
 
 AVCodec ff_screenpresso_decoder = {