Fix MB skipping with droppable frames.

Fixes issue323.

Originally committed as revision 13378 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo.c

@@ -236,7 +236,7 @@ int alloc_picture(MpegEncContext *s, Picture *pic, int shared){
     /* It might be nicer if the application would keep track of these
      * but it would require an API change. */
     memmove(s->prev_pict_types+1, s->prev_pict_types, PREV_PICT_TYPES_BUFFER_SIZE-1);
-    s->prev_pict_types[0]= s->pict_type;
+    s->prev_pict_types[0]= s->dropable ? FF_B_TYPE : s->pict_type;
     if(pic->age < PREV_PICT_TYPES_BUFFER_SIZE && s->prev_pict_types[pic->age] == FF_B_TYPE)
         pic->age= INT_MAX; // Skipped MBs in B-frames are quite rare in MPEG-1/2 and it is a bit tricky to skip them anyway.