Fix decoding of DXSA subtitles.

These have a DXSA tag and contain alpha in addition to
color values for palette.

libavcodec/xsubdec.c

@@ -54,6 +54,7 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size,
     int w, h, x, y, rlelen, i;
     int64_t packet_time = 0;
     GetBitContext gb;
+    int has_alpha = avctx->codec_tag == MKTAG('D','X','S','A');
 
     // check that at least header fits
     if (buf_size < 27 + 7 * 2 + 4 * 3) {
@@ -100,8 +101,8 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size,
     for (i = 0; i < sub->rects[0]->nb_colors; i++)
         ((uint32_t*)sub->rects[0]->pict.data[1])[i] = bytestream_get_be24(&buf);
     // make all except background (first entry) non-transparent
-    for (i = 1; i < sub->rects[0]->nb_colors; i++)
-        ((uint32_t*)sub->rects[0]->pict.data[1])[i] |= 0xff000000;
+    for (i = 0; i < sub->rects[0]->nb_colors; i++)
+        ((uint32_t*)sub->rects[0]->pict.data[1])[i] |= (has_alpha ? *buf++ : (i ? 0xff : 0)) << 24;
 
     // process RLE-compressed data
     rlelen = FFMIN(rlelen, buf_end - buf);