player: fix --lavfi-complex freeze

Commit 0e0b87b6f3 fixed that dropped packets did not trigger further
work correctly. But it also made trivial --lavfi-complex freeze. The
reason is that the meaning if DATA_AGAIN was overloaded: the decoders
meant that they should be called again, while lavfi.c meant that other
outputs needed to be checked again. Rename the latter meaning to
DATA_STARVE, which means that the current input will deliver no more
data, until "other" work has been done (like reading other outputs, or
feeding input).

The decoders never return DATA_STARVE, because they don't get input from
the player core (instead, they get it from the demuxer directly, which
is why they still can return DATA_WAIT).

Also document the DATA_* semantics in the enum.

Fixes #4746.

player/video.c

@@ -617,6 +617,7 @@ static int decode_image(struct MPContext *mpctx)
     switch (res) {
     case DATA_WAIT:     return VD_WAIT;
     case DATA_OK:
+    case DATA_STARVE:
     case DATA_AGAIN:    return VD_PROGRESS;
     case DATA_EOF:      return VD_EOF;
     default:            abort();