command: send additional property change events in-between loading

Opening the stream and opening the demuxer are both done asynchronously,
meaning the player reacts to client API requests. They also can
potentially take a while. Thus it's better to process outstanding
property changes, so that change events are sent for properties that
were changed during opening.

player/command.h

@@ -43,6 +43,7 @@ enum {
     // Must start with the first unused positive value in enum mpv_event_id
     // MPV_EVENT_* and MP_EVENT_* must not overlap.
     INTERNAL_EVENT_BASE = 25,
+    MP_EVENT_CHANGE_ALL,
     MP_EVENT_CACHE_UPDATE,
     MP_EVENT_WIN_RESIZE,
     MP_EVENT_WIN_STATE,