Fix lowres > 0 with --enable-avfilter..

Fixes issue 2072 and issue 2421.

Patch by Anatoly Nenashev, anatoly d nenashev a ovsoft d ru

Originally committed as revision 26252 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -3274,6 +3274,8 @@ static void opt_input_file(const char *filename)
                 dec->flags |= CODEC_FLAG_EMU_EDGE;
                 frame_height >>= dec->lowres;
                 frame_width  >>= dec->lowres;
+                dec->height = frame_height;
+                dec->width  = frame_width;
             }
             if(me_threshold)
                 dec->debug |= FF_DEBUG_MV;