add png fourcc

Originally committed as revision 7540 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mov.c

@@ -122,6 +122,7 @@ static const CodecTag mov_video_tags[] = {
     { CODEC_ID_RAWVIDEO, MKTAG('2', 'v', 'u', 'y') }, /* UNCOMPRESSED 8BIT 4:2:2 */
     { CODEC_ID_VC1,  MKTAG('v', 'c', '-', '1') }, /* SMPTE RP 2025 */
     { CODEC_ID_WMV3, MKTAG('v', 'c', '-', '1') }, /* SMPTE RP 2025 */
+    { CODEC_ID_PNG,  MKTAG('p', 'n', 'g', ' ') },
     { CODEC_ID_NONE, 0 },
 };