Implement FFMAX3(a,b,c) - maximum over three arguments.

Originally committed as revision 12910 to svn://svn.ffmpeg.org/ffmpeg/trunk
diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 321968a..25fbdd5 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -98,8 +98,8 @@
     //..
 } MatroskaSubtitleTrack;
 
-#define MAX_TRACK_SIZE (FFMAX(FFMAX(sizeof(MatroskaVideoTrack), \
-                                    sizeof(MatroskaAudioTrack)), \
+#define MAX_TRACK_SIZE (FFMAX3(sizeof(MatroskaVideoTrack), \
+                                    sizeof(MatroskaAudioTrack), \
                                     sizeof(MatroskaSubtitleTrack)))
 
 typedef struct MatroskaLevel {