Port overlay filter from the libavfilter repo (with many fixes),

adopting Baptiste variant which is simpler and faster.

Originally committed as revision 25784 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavfilter/allfilters.c

@@ -52,6 +52,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER (NOFORMAT,    noformat,    vf);
     REGISTER_FILTER (NULL,        null,        vf);
     REGISTER_FILTER (OCV_SMOOTH,  ocv_smooth,  vf);
+    REGISTER_FILTER (OVERLAY,     overlay,     vf);
     REGISTER_FILTER (PAD,         pad,         vf);
     REGISTER_FILTER (PIXDESCTEST, pixdesctest, vf);
     REGISTER_FILTER (PIXELASPECT, pixelaspect, vf);