Fix timestamp scaling in the overlay filter.
Compute the outpicref PTS in start_frame(), since it is required forcomparing that value with the value of the overpicref.
Originally committed as revision 25828 to svn://svn.ffmpeg.org/ffmpeg/trunk
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