demux_lavf: mark as seekable if protocol supports seeking by time

Basically, this will mark the demuxer as seekable with rtmp* and mmsh
protocols. These protocols have network-level time seeking, and whether
you can seek on the byte level does not matter.

Until now, seeking was typically only enabled because of the cache, and
a (nonsensical) warning was shown accordingly.

It still could happen that the server doesn't actually support thse
requests (or simply rejects them), so this is somewhat imperfect.

demux/demux_lavf.c

@@ -729,6 +729,8 @@ static int demux_open_lavf(demuxer_t *demuxer, enum demux_check check)
         priv->pb->read_seek = mp_read_seek;
         priv->pb->seekable = demuxer->seekable ? AVIO_SEEKABLE_NORMAL : 0;
         avfc->pb = priv->pb;
+        if (stream_control(demuxer->stream, STREAM_CTRL_HAS_AVSEEK, NULL) > 0)
+            demuxer->seekable = true;
     }
 
     if (matches_avinputformat_name(priv, "rtsp")) {

stream/cache.c

@@ -112,6 +112,7 @@ struct priv {
     int64_t stream_size;
     struct mp_tags *stream_metadata;
     double start_pts;
+    bool has_avseek;
 };
 
 enum {
@@ -361,6 +362,7 @@ static void update_cached_controls(struct priv *s)
     s->stream_size = -1;
     if (stream_control(s->stream, STREAM_CTRL_GET_SIZE, &i64) == STREAM_OK)
         s->stream_size = i64;
+    s->has_avseek = stream_control(s->stream, STREAM_CTRL_HAS_AVSEEK, NULL) > 0;
 }
 
 // the core might call these every frame, so cache them...
@@ -391,6 +393,8 @@ static int cache_get_cached_control(stream_t *cache, int cmd, void *arg)
         *(double *)arg = s->start_pts;
         return STREAM_OK;
     }
+    case STREAM_CTRL_HAS_AVSEEK:
+        return s->has_avseek ? STREAM_OK : STREAM_UNSUPPORTED;
     case STREAM_CTRL_GET_METADATA: {
         if (s->stream_metadata) {
             ta_set_parent(s->stream_metadata, NULL);

stream/stream.h

@@ -105,6 +105,7 @@ enum stream_ctrl {
     STREAM_CTRL_DVB_SET_CHANNEL,
     STREAM_CTRL_DVB_STEP_CHANNEL,
     STREAM_CTRL_AVSEEK,
+    STREAM_CTRL_HAS_AVSEEK,
 };
 
 struct stream_lang_req {

stream/stream_lavf.c

@@ -117,6 +117,10 @@ static int control(stream_t *s, int cmd, void *arg)
             return 1;
         break;
     }
+    case STREAM_CTRL_HAS_AVSEEK:
+        if (avio->read_seek)
+            return 1;
+        break;
     case STREAM_CTRL_GET_METADATA: {
         *(struct mp_tags **)arg = read_icy(s);
         if (!*(struct mp_tags **)arg)