video: allow the decoder to consume packets partially

This is in preparation for a hypothetical API change in libavcodec,
which would allow the decoder to return multiple video frames before
accepting a new input packet.

In theory, the body of the if() added to vd_lavc.c could be replaced
with this code:

    packet->buffer += ret;
    packet->len    -= ret;

but currently this is not needed, as libavformat already outputs one
frame per packet. Also, using libavcodec this way could lead to a
"deadlock" if the decoder refuses to consume e.g. garbage padding, so
enabling this now would introduce bugs.

(Adding this now for easier testing, and for symmetry with the audio
code.)

video/decode/dec_video.c

@@ -412,8 +412,10 @@ void video_work(struct dec_video *d_video)
         framedrop_type = 2;
     }
     d_video->current_mpi = decode_packet(d_video, d_video->packet, framedrop_type);
-    talloc_free(d_video->packet); // always fully consumed
-    d_video->packet = NULL;
+    if (d_video->packet && d_video->packet->len == 0) {
+        talloc_free(d_video->packet);
+        d_video->packet = NULL;
+    }
 
     d_video->current_state = DATA_OK;
     if (!d_video->current_mpi) {

video/decode/vd_lavc.c

@@ -702,6 +702,11 @@ static void decode(struct dec_video *vd, struct demux_packet *packet,
         return;
     }
 
+    if (packet) {
+        // always fully consumed
+        packet->len = 0;
+    }
+
     // Skipped frame, or delayed output due to multithreaded decoding.
     if (!got_picture) {
         if (!packet)