mpegvideo: claim ownership of referenced pictures

Under certain conditions pictures could be released before they were
returned with frame-threading. Broken mv computation in the upcoming
rv34 frame-threading patch was caused by this.

To prevent contexts from running out of available pictures the loop
releasing "unused" pictures has to be run for B frames too.

libavcodec/mpegvideo.c

@@ -1170,18 +1170,20 @@ int MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
            s->codec_id == CODEC_ID_SVQ3);
 
     /* mark & release old frames */
+    if (s->out_format != FMT_H264 || s->codec_id == CODEC_ID_SVQ3) {
         if (s->pict_type != AV_PICTURE_TYPE_B && s->last_picture_ptr &&
             s->last_picture_ptr != s->next_picture_ptr &&
             s->last_picture_ptr->f.data[0]) {
-        if (s->out_format != FMT_H264 || s->codec_id == CODEC_ID_SVQ3) {
             if (s->last_picture_ptr->owner2 == s)
                 free_frame_buffer(s, s->last_picture_ptr);
+        }
 
         /* release forgotten pictures */
         /* if (mpeg124/h263) */
         if (!s->encoding) {
             for (i = 0; i < s->picture_count; i++) {
                 if (s->picture[i].owner2 == s && s->picture[i].f.data[0] &&
+                    &s->picture[i] != s->last_picture_ptr &&
                     &s->picture[i] != s->next_picture_ptr &&
                     s->picture[i].f.reference) {
                     if (!(avctx->active_thread_type & FF_THREAD_FRAME))
@@ -1192,7 +1194,6 @@ int MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             }
         }
     }
-    }
 
     if (!s->encoding) {
         ff_release_unused_pictures(s, 1);
@@ -1295,6 +1296,14 @@ int MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
     if (s->next_picture_ptr)
         ff_copy_picture(&s->next_picture, s->next_picture_ptr);
 
+    if (HAVE_THREADS && (avctx->active_thread_type & FF_THREAD_FRAME) &&
+        (s->out_format != FMT_H264 || s->codec_id == CODEC_ID_SVQ3)) {
+        if (s->next_picture_ptr)
+            s->next_picture_ptr->owner2 = s;
+        if (s->last_picture_ptr)
+            s->last_picture_ptr->owner2 = s;
+    }
+
     assert(s->pict_type == AV_PICTURE_TYPE_I || (s->last_picture_ptr &&
                                                  s->last_picture_ptr->f.data[0]));