Allocate AVPacket for stream->pending

From FFmpeg/doc/APIchanges:

    2021-03-17 - f7db77bd87 - lavc 58.133.100 - codec.h
      Deprecated av_init_packet(). Once removed, sizeof(AVPacket) will
      no longer be a part of the public ABI.

Remove the has_pending boolean, which can be replaced by:

    stream->pending != NULL

Refs #2302 <https://github.com/Genymobile/scrcpy/issues/2302>

app/src/stream.h

@@ -24,8 +24,7 @@ struct stream {
     AVCodecParserContext *parser;
     // successive packets may need to be concatenated, until a non-config
     // packet is available
-    bool has_pending;
-    AVPacket pending;
+    AVPacket *pending;
 
     const struct stream_callbacks *cbs;
     void *cbs_userdata;