Fix red/blue swap for 4xa files.

Originally committed as revision 21070 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/4xm.c

@@ -815,7 +815,7 @@ static av_cold int decode_init(AVCodecContext *avctx){
     init_vlcs(f);
 
     if(f->version>2) avctx->pix_fmt= PIX_FMT_RGB565;
-    else             avctx->pix_fmt= PIX_FMT_RGB555;
+    else             avctx->pix_fmt= PIX_FMT_BGR555;
 
     return 0;
 }