avformat/img2dec: reduce bmppipe probe score

bmp pipe needs the bmp parser which is not bug free and should thus not be favored
over the bmp image2 demuxer
that also means this change could be reverted in case bmp pipe is improved so it
handles all single bmp images correctly

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/img2dec.c

@@ -574,7 +574,7 @@ static int bmp_probe(AVProbeData *p)
         return 0;
 
     if (!AV_RN32(b + 6)) {
-        return AVPROBE_SCORE_EXTENSION + 1;
+        return AVPROBE_SCORE_EXTENSION - 1; // lower than extension as bmp pipe has bugs
     } else {
         return AVPROBE_SCORE_EXTENSION / 4;
     }