player: warn if PTS association mode switches

This normally shouldn't happen. It does happen with VfW-muxed mkv files,
and would normally happen with avi files (except that we force the
correct association mode using an explicit hack for avi).

This is usually prepended by warnings like:

 Decreasing video pts: 0.125000 < 0.167000

and after the switch, there should be no warnings anymore.

Background: avi likes to use DTS for timestamps instead of PTS.
Basically, there are no proper timestamps in the file, only frame
numbers. And Matroska is insane and stores the made-up DTS instead
of a proper PTS. This will be handled properly later.

mpvcore/player/video.c

@@ -374,8 +374,8 @@ static void determine_frame_pts(struct MPContext *mpctx)
         }
         if (probcount1 >= probcount2 * 1.5 + 2) {
             d_video->pts_assoc_mode = 3 - d_video->pts_assoc_mode;
-            MP_VERBOSE(mpctx, "Switching to pts association mode "
-                       "%d.\n", d_video->pts_assoc_mode);
+            MP_WARN(mpctx, "Switching to pts association mode "
+                    "%d.\n", d_video->pts_assoc_mode);
         }
     }
     d_video->pts = d_video->pts_assoc_mode == 1 ?