filter_blend, multicat_sink: enchance debug
add ubuf_pic clear function

- add ubuf_pic_plane_clear, ubuf_pic_clear,
  and corresponding uref_pic_*.
- clear picture in upipe_sws_thumbs

TODO: support for u16*/v16* and packed YUV
upipe-avcenc: allow setting codec by name
upipe-avcdec: set real pic size in output flow