ffmpeg_filter: Do not override the dimensions in sub2video_prepare()

Fixes ticket4744 part1
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

ffmpeg_filter.c

@@ -682,8 +682,8 @@ static int sub2video_prepare(InputStream *ist)
         }
         av_log(avf, AV_LOG_INFO, "sub2video: using %dx%d canvas\n", w, h);
     }
-    ist->sub2video.w = ist->dec_ctx->width  = ist->resample_width  = w;
-    ist->sub2video.h = ist->dec_ctx->height = ist->resample_height = h;
+    ist->sub2video.w = ist->resample_width  = w;
+    ist->sub2video.h = ist->resample_height = h;
 
     /* rectangles are AV_PIX_FMT_PAL8, but we have no guarantee that the
        palettes for all rectangles are identical or compatible */