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.