player: seek backward for type MPSEEK_FACTOR

Choosing the seek direction for MPSEEK_FACTOR based on the location of
seek_pts is nonsense, and can cause the seek position to oscillate
between adjacent keyframes, such as when dragging the OSC bar on short
videos.

Fix this by always seeking backward for type MPSEEK_FACTOR, i.e. seek
to the keyframe for the group of pictures (GOP) in which seek_pts is
located.

Fixes #4183.

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

player/playloop.c

@@ -284,7 +284,7 @@ static void mp_seek(MPContext *mpctx, struct seek_params seek)
         double len = get_time_length(mpctx);
         if (len >= 0)
             seek_pts = seek.amount * len;
-        demux_flags = seek_pts > current_time ? SEEK_FORWARD : SEEK_BACKWARD;
+        demux_flags = SEEK_BACKWARD;
         break;
     default: abort();
     }