xsub: Support DXSA subtitles

These have a DXSA tag and contain alpha in addition to
color values for palette.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/xsubdec.c

@@ -56,11 +56,12 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size,
     int w, h, x, y, i;
     int64_t packet_time = 0;
     GetBitContext gb;
+    int has_alpha = avctx->codec_tag == MKTAG('D','X','S','A');
 
     memset(sub, 0, sizeof(*sub));
 
     // check that at least header fits
-    if (buf_size < 27 + 7 * 2 + 4 * 3) {
+    if (buf_size < 27 + 7 * 2 + 4 * (3 + has_alpha)) {
         av_log(avctx, AV_LOG_ERROR, "coded frame too small\n");
         return -1;
     }
@@ -107,9 +108,15 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size,
     // read palette
     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;
+
+    if (!has_alpha) {
+        // 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;
+    } else {
+        for (i = 0; i < sub->rects[0]->nb_colors; i++)
+            ((uint32_t *)sub->rects[0]->pict.data[1])[i] |= *buf++ << 24;
+    }
 
     // process RLE-compressed data
     init_get_bits(&gb, buf, (buf_end - buf) * 8);