rate limit tips and checks

Originally committed as revision 2643 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo.c

@@ -731,6 +731,15 @@ int MPV_encode_init(AVCodecContext *avctx)
     s->loop_filter= !!(s->flags & CODEC_FLAG_LOOP_FILTER);
     s->alternate_scan= !!(s->flags & CODEC_FLAG_ALT_SCAN);
 
+    if(avctx->rc_max_rate && !avctx->rc_buffer_size){
+        av_log(avctx, AV_LOG_ERROR, "a vbv buffer size is needed, for encoding with a maximum bitrate\n");
+        return -1;
+    }    
+
+    if(avctx->rc_min_rate && avctx->rc_max_rate != avctx->rc_min_rate){
+        av_log(avctx, AV_LOG_INFO, "Warning min_rate > 0 but min_rate != max_rate isnt recommanded!\n");
+    }    
+        
     if((s->flags & CODEC_FLAG_4MV) && s->codec_id != CODEC_ID_MPEG4 
        && s->codec_id != CODEC_ID_H263 && s->codec_id != CODEC_ID_H263P){
         av_log(avctx, AV_LOG_ERROR, "4MV not supported by codec\n");