add little endian 24bit read/write

Originally committed as revision 8270 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/bytestream.h

@@ -33,6 +33,7 @@ static av_always_inline void bytestream_put_ ##name(uint8_t **b, const unsigned
 };
 
 DEF(le32, 4, AV_RL32, AV_WL32)
+DEF(le24, 3, AV_RL24, AV_WL24)
 DEF(le16, 2, AV_RL16, AV_WL16)
 DEF(be32, 4, AV_RB32, AV_WB32)
 DEF(be24, 3, AV_RB24, AV_WB24)