demux_lavf: add hack to workaround too unreliable mp3 detection

This generally affects mp3 files that don't have any (or many) mp3
frames in the first 2 MB. 2 MB is the maximum probe size, and
libavformat returns a low probescore even if we give it the full 2 MB.
Trying to probe a larger buffer (or even the full file) doesn't work for
mysterious reasons.

The workaround consists in accepting a very weak probescore if the
format is detected as mp3 and we probed already 2 MB.

demux/demux_lavf.c

@@ -98,6 +98,8 @@ static const struct format_hack format_hacks[] = {
     {"aac", "audio/aacp", 25, 0.5},
     {"aac", "audio/aac",  25, 0.5},
     {"mp3", "audio/mpeg", 25, 0.5},
+    // some mp3 files don't detect correctly
+    {"mp3", NULL,         24, .max_probe = true},
     {0}
 };