blend filter

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/allfilters.c

@@ -94,6 +94,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER(BBOX,           bbox,           vf);
     REGISTER_FILTER(BLACKDETECT,    blackdetect,    vf);
     REGISTER_FILTER(BLACKFRAME,     blackframe,     vf);
+    REGISTER_FILTER(BLEND,          blend,          vf);
     REGISTER_FILTER(BOXBLUR,        boxblur,        vf);
     REGISTER_FILTER(COLORMATRIX,    colormatrix,    vf);
     REGISTER_FILTER(COPY,           copy,           vf);