avcodec: add ARBC decoder

Thanks Kostya for great help in reversing binary.

libavformat/riff.c

@@ -476,6 +476,7 @@ const AVCodecTag ff_codec_bmp_tags[] = {
     { AV_CODEC_ID_WCMV,         MKTAG('W', 'C', 'M', 'V') },
     { AV_CODEC_ID_RASC,         MKTAG('R', 'A', 'S', 'C') },
     { AV_CODEC_ID_HYMT,         MKTAG('H', 'Y', 'M', 'T') },
+    { AV_CODEC_ID_ARBC,         MKTAG('A', 'R', 'B', 'C') },
     { AV_CODEC_ID_NONE,         0 }
 };