reuses a given picture_t without allocating a new one.
* modules/video_output/picture.c: New --picture-height and width options
  to do the scaling in the vout thread. Misc bug corrections.
* modules/video_filter/mosaic.c: Less overhead.
* modules/codec/ffmpeg/video_filter.c: Do the scaling before the conversion
  in case the original size is bigger than the destination.