ffmpeg: disable dr1 when frame sizes change.

dr1 seems to work fine with frame size changes but many filters
cant handle it yet. Simply disabling it forces the alternative
non dr1 code path which has been tested more completely and
is known to handle frame size changes in a wider varity of
cases.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -2174,10 +2174,13 @@ static int transcode_video(InputStream *ist, AVPacket *pkt, int *got_output, int
     for(i=0;i<nb_output_streams;i++) {
         OutputStream *ost = ost = &output_streams[i];
         if(check_output_constraints(ist, ost) && ost->encoding_needed){
+            int changed =    ist->st->codec->width   != ost->input_video_filter->outputs[0]->w
+                          || ist->st->codec->height  != ost->input_video_filter->outputs[0]->h
+                          || ist->st->codec->pix_fmt != ost->input_video_filter->outputs[0]->format;
             if (!frame_sample_aspect->num)
                 *frame_sample_aspect = ist->st->sample_aspect_ratio;
             decoded_frame->pts = ist->pts;
-            if (ist->dr1 && decoded_frame->type==FF_BUFFER_TYPE_USER) {
+            if (ist->dr1 && decoded_frame->type==FF_BUFFER_TYPE_USER && !changed) {
                 FrameBuffer      *buf = decoded_frame->opaque;
                 AVFilterBufferRef *fb = avfilter_get_video_buffer_ref_from_arrays(
                                             decoded_frame->data, decoded_frame->linesize,