command: add track-list/N/audio-channels property

Signed-off-by: wm4 <wm4@nowhere>

(With some heavy modifications from the original patch.)

DOCS/interface-changes.rst

@@ -20,6 +20,7 @@ Interface changes
 ::
 
  --- mpv 0.10.0 will be released ---
+    - add ``track-list/N/audio-channels`` property
     - change --screenshot-tag-colorspace default value
     - add --stretch-image-subs-to-screen
     - add "playlist/N/title" property

DOCS/man/input.rst

@@ -1574,6 +1574,10 @@ Property list
     ``track-list/N/lang``
         Track language as identified by the file. Not always available.
 
+    ``track-list/N/audio-channels``
+        For audio tracks, the number of audio channels in the audio stream.
+        Not always accurate (depends on container hints). Not always available.
+
     ``track-list/N/albumart``
         ``yes`` if this is a video track that consists of a single picture,
         ``no`` or unavailable otherwise. This is used for video tracks that are
@@ -1622,6 +1626,7 @@ Property list
                 "src-id"            MPV_FORMAT_INT64
                 "title"             MPV_FORMAT_STRING
                 "lang"              MPV_FORMAT_STRING
+                "audio-channels"    MPV_FORMAT_INT64
                 "albumart"          MPV_FORMAT_FLAG
                 "default"           MPV_FORMAT_FLAG
                 "forced"            MPV_FORMAT_FLAG