mpegvideo_enc: enable rtp_mode when multiple slices are used

Currently, multiple slices with just one thread produce corrupted
output.

Additionally, enable slice structured mode for h263(+)

Bug-Id: 912
CC: libav-stabl@libav.org

libavcodec/mpegvideo_enc.c

@@ -519,9 +519,6 @@ FF_ENABLE_DEPRECATION_WARNINGS
         return -1;
     }
 
-    if (s->avctx->thread_count > 1)
-        s->rtp_mode = 1;
-
     if (!avctx->time_base.den || !avctx->time_base.num) {
         av_log(avctx, AV_LOG_ERROR, "framerate not set\n");
         return -1;
@@ -764,6 +761,13 @@ FF_ENABLE_DEPRECATION_WARNINGS
     if ((CONFIG_H263P_ENCODER || CONFIG_RV20_ENCODER) && s->modified_quant)
         s->chroma_qscale_table = ff_h263_chroma_qscale_table;
 
+    if (s->slice_context_count > 1) {
+        s->rtp_mode = 1;
+
+        if (avctx->codec_id == AV_CODEC_ID_H263 || avctx->codec_id == AV_CODEC_ID_H263P)
+            s->h263_slice_structured = 1;
+    }
+
     s->quant_precision = 5;
 
     ff_set_cmp(&s->mecc, s->mecc.ildct_cmp,      s->avctx->ildct_cmp);