add support for AVI files with On2 header

Originally committed as revision 10591 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avidec.c

@@ -61,6 +61,7 @@ static const char avi_headers[][8] = {
     { 'R', 'I', 'F', 'F',    'A', 'V', 'I', ' ' },
     { 'R', 'I', 'F', 'F',    'A', 'V', 'I', 'X' },
     { 'R', 'I', 'F', 'F',    'A', 'V', 'I', 0x19},
+    { 'O', 'N', '2', ' ',    'O', 'N', '2', 'f' },
     { 0 }
 };