dxva2_h264: set the correct ref frame index in the long slice struct

The latest H.264 DXVA specification states that the index in this
structure should refer to a valid entry in the RefFrameList of the picture
parameter structure, and not to the actual surface index.

Fixes H.264 DXVA2 decoding on recent Intel GPUs (tested on Sandy and Ivy)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/dxva2_h264.c

@@ -195,8 +195,18 @@ static void fill_slice_short(DXVA_Slice_H264_Short *slice,
     slice->wBadSliceChopping     = 0;
 }
 
+static int get_refpic_index(const DXVA_PicParams_H264 *pp, int surface_index)
+{
+    int i;
+    for (i = 0; i < FF_ARRAY_ELEMS(pp->RefFrameList); i++) {
+        if ((pp->RefFrameList[i].bPicEntry & 0x7f) == surface_index)
+          return i;
+    }
+    return 0x7f;
+}
+
 static void fill_slice_long(AVCodecContext *avctx, DXVA_Slice_H264_Long *slice,
-                            unsigned position, unsigned size)
+                            const DXVA_PicParams_H264 *pp, unsigned position, unsigned size)
 {
     const H264Context *h = avctx->priv_data;
     struct dxva_context *ctx = avctx->hwaccel_context;
@@ -229,8 +239,8 @@ static void fill_slice_long(AVCodecContext *avctx, DXVA_Slice_H264_Long *slice,
             if (list < h->list_count && i < h->ref_count[list]) {
                 const H264Picture *r = &h->ref_list[list][i];
                 unsigned plane;
-                fill_picture_entry(&slice->RefPicList[list][i],
-                                   ff_dxva2_get_surface_index(ctx, &r->f),
+                unsigned index = get_refpic_index(pp, ff_dxva2_get_surface_index(ctx, &r->f));
+                fill_picture_entry(&slice->RefPicList[list][i], index,
                                    r->reference == PICT_BOTTOM_FIELD);
                 for (plane = 0; plane < 3; plane++) {
                     int w, o;
@@ -415,7 +425,7 @@ static int dxva2_h264_decode_slice(AVCodecContext *avctx,
                          position, size);
     else
         fill_slice_long(avctx, &ctx_pic->slice_long[ctx_pic->slice_count],
-                        position, size);
+                        &ctx_pic->pp, position, size);
     ctx_pic->slice_count++;
 
     if (h->slice_type != AV_PICTURE_TYPE_I && h->slice_type != AV_PICTURE_TYPE_SI)