h264: start splitting decode_slice_header()

That function is currently very long and entangles bitstream parsing and
decoder configuration. This makes the code much harder to read than
necessary.

Begin splitting the code that configures the decoder state based on the
slice header information from the parsing of the slice header.

libavcodec/h264_slice.c

@@ -901,22 +901,14 @@ static int h264_slice_header_init(H264Context *h)
     return 0;
 }
 
-/**
- * Decode a slice header.
- * This will (re)initialize the decoder and call h264_frame_start() as needed.
- *
- * @param h h264context
- *
- * @return 0 if okay, <0 if an error occurred
- */
-int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
+static int h264_slice_header_parse(H264Context *h, H264SliceContext *sl)
 {
     const SPS *sps;
     const PPS *pps;
     unsigned int first_mb_in_slice;
     unsigned int pps_id;
     int ret;
-    unsigned int slice_type, tmp, i, j;
+    unsigned int slice_type, tmp, i;
     int last_pic_structure, last_pic_droppable;
     int needs_reinit = 0;
     int field_pic_flag, bottom_field_flag;
@@ -1453,6 +1445,25 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
         }
     }
 
+    return 0;
+}
+
+/**
+ * Decode a slice header.
+ * This will (re)initialize the decoder and call h264_frame_start() as needed.
+ *
+ * @param h h264context
+ *
+ * @return 0 if okay, <0 if an error occurred
+ */
+int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
+{
+    int i, j, ret = 0;
+
+    ret = h264_slice_header_parse(h, sl);
+    if (ret < 0)
+        return ret;
+
     if (h->avctx->skip_loop_filter >= AVDISCARD_ALL ||
         (h->avctx->skip_loop_filter >= AVDISCARD_NONKEY &&
          sl->slice_type_nos != AV_PICTURE_TYPE_I) ||
@@ -1474,9 +1485,9 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
     sl->qp_thresh = 15 -
                    FFMIN(sl->slice_alpha_c0_offset, sl->slice_beta_offset) -
                    FFMAX3(0,
-                          pps->chroma_qp_index_offset[0],
-                          pps->chroma_qp_index_offset[1]) +
-                   6 * (sps->bit_depth_luma - 8);
+                          h->ps.pps->chroma_qp_index_offset[0],
+                          h->ps.pps->chroma_qp_index_offset[1]) +
+                   6 * (h->ps.sps->bit_depth_luma - 8);
 
     sl->slice_num       = ++h->current_slice;
     if (sl->slice_num >= MAX_SLICES) {
@@ -1519,14 +1530,14 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
 
     if (h->avctx->debug & FF_DEBUG_PICT_INFO) {
         av_log(h->avctx, AV_LOG_DEBUG,
-               "slice:%d %s mb:%d %c%s%s pps:%u frame:%d poc:%d/%d ref:%d/%d qp:%d loop:%d:%d:%d weight:%d%s %s\n",
+               "slice:%d %s mb:%d %c%s%s frame:%d poc:%d/%d ref:%d/%d qp:%d loop:%d:%d:%d weight:%d%s %s\n",
                sl->slice_num,
                (h->picture_structure == PICT_FRAME ? "F" : h->picture_structure == PICT_TOP_FIELD ? "T" : "B"),
-               first_mb_in_slice,
+               sl->mb_y * h->mb_width + sl->mb_x,
                av_get_picture_type_char(sl->slice_type),
                sl->slice_type_fixed ? " fix" : "",
                h->nal_unit_type == NAL_IDR_SLICE ? " IDR" : "",
-               pps_id, h->poc.frame_num,
+               h->poc.frame_num,
                h->cur_pic_ptr->field_poc[0],
                h->cur_pic_ptr->field_poc[1],
                sl->ref_count[0], sl->ref_count[1],