command: change stream-pos semantics

Changing the byte stream position without cooperation of the demuxer
seems a bit insane, and is certainly useless. A user should do factor
seeks instead. For formats like ts, this will actually translate to byte
seeks, while treating the rest of the playback chain a bit more
gracefully. With this argument, remove write access to this property.

If someone really complains, proper byte seeks could be added as seek
mode (although I'm going to need a convincing argument for this).

Read access changes too, but in a more subtle way.

DOCS/interface-changes.rst

@@ -22,6 +22,7 @@ Interface changes
  --- mpv 0.17.0 ---
     - deprecate "track-list/N/audio-channels" property (use
       "track-list/N/demux-channel-count" instead)
+    - remove write access to "stream-pos", and change semantics for read access
  --- mpv 0.16.0 ---
     - change --audio-channels default to stereo (use --audio-channels=auto to
       get the old default)

DOCS/man/input.rst

@@ -921,8 +921,9 @@ Property list
     useless. It looks like this can be different from ``path`` only when
     using e.g. ordered chapters.)
 
-``stream-pos`` (RW)
-    Raw byte position in source stream.
+``stream-pos``
+    Raw byte position in source stream. Technically, this returns the position
+    of the most recent packet passed to a decoder.
 
 ``stream-end``
     Raw end position in bytes in source stream.

player/command.c

@@ -458,24 +458,14 @@ static int mp_property_file_format(void *ctx, struct m_property *prop,
     return m_property_strdup_ro(action, arg, name);
 }
 
-/// Position in the stream (RW)
 static int mp_property_stream_pos(void *ctx, struct m_property *prop,
                                   int action, void *arg)
 {
     MPContext *mpctx = ctx;
     struct demuxer *demuxer = mpctx->demuxer;
-    if (!demuxer)
+    if (!demuxer || demuxer->filepos < 0)
         return M_PROPERTY_UNAVAILABLE;
-    demux_pause(demuxer);
-    int r;
-    if (action == M_PROPERTY_SET) {
-        stream_seek(demuxer->stream, *(int64_t *) arg);
-        r = M_PROPERTY_OK;
-    } else {
-        r = m_property_int64_ro(action, arg, stream_tell(demuxer->stream));
-    }
-    demux_unpause(demuxer);
-    return r;
+    return m_property_int64_ro(action, arg, demuxer->filepos);
 }
 
 /// Stream end offset (RO)