player: flush decoder even if cover art is decoded

Fixes PNG cover art not showing up immediately (for example when running
with --pause).

libavformat exports embedded cover art as a single packet. For example,
a PNG attachment simply contains the PNG image, which can be sent to the
decoder. Normally you would expect that the PNG decoder would return 1
frame for 1 packet, without any delays. But this stopped working, and it
incurs a 1 frame delay.

This is perfectly legal (even if unexpected), so let our code feed the
decoder packets until we get something back. (In theory feeding the
packet instead of a real flush packet is still somewhat questionable.)

player/video.c

@@ -377,7 +377,7 @@ static int decode_image(struct MPContext *mpctx)
     if (d_video->header->attached_picture) {
         d_video->waiting_decoded_mpi =
                     video_decode(d_video, d_video->header->attached_picture, 0);
-        return VD_EOF;
+        return d_video->waiting_decoded_mpi ? VD_EOF : VD_PROGRESS;
     }
 
     struct demux_packet *pkt;